テック英語：デジタル産業の必須語彙ガイド

"refactor"

To restructure existing code without changing its external behaviour, improving readability, performance, or maintainability

"The tech lead asked the team to refactor the authentication module before adding new features, as the existing code had grown too complex to extend safely."

"legacy code"

Older code that is still in production but is difficult to modify because it lacks tests, documentation, or uses outdated patterns

"Onboarding new engineers took weeks because the legacy code had no documentation and relied on undocumented side effects that were impossible to reason about."

"code review"

A collaborative process where one or more engineers examine another engineer's proposed changes before they are merged into the main codebase

"The engineering culture required every pull request to pass a code review from at least two senior engineers before merging, which caught three critical security bugs in one quarter."

"merge conflict"

A situation that occurs when two branches have made incompatible changes to the same file, requiring manual resolution before the branches can be combined

"Working on the same configuration file for two weeks caused a merge conflict that took the team an afternoon to resolve without losing any changes from either branch."

"dependency injection"

A design pattern where a component receives its dependencies from an external source rather than creating them itself, making the code more testable and modular

"Switching to dependency injection meant the payment service could be unit-tested in complete isolation by passing in a mock bank client instead of calling the real payment gateway."

"singleton pattern"

A creational design pattern that ensures a class has only one instance and provides a global point of access to it, commonly used for database connections or configuration objects

"The developer used a singleton pattern for the database connection pool so that all parts of the application shared the same pool without creating duplicate connections."

"dead code"

Code that is never executed because it is unreachable or because the feature it supports has been removed, which increases codebase complexity without providing value

"The static analysis tool identified 1,200 lines of dead code in the repository, left over from three features that were disabled two years earlier but never fully removed."

"code smell"

A surface-level indicator in source code that suggests an underlying design problem, such as overly long methods, duplicated logic, or excessive class coupling

"The reviewer left a comment flagging three code smells in the pull request — a 300-line function, duplicated validation logic, and a class that accessed another class's private state directly."

"spin up"

To launch or start a new server, container, virtual machine, or cloud environment, often used informally in engineering conversations

"The DevOps engineer could spin up a complete staging environment in under ten minutes thanks to the Terraform scripts and automated provisioning pipeline."

"auto-scaling"

A cloud feature that automatically adjusts the number of running instances of an application based on current demand, ensuring performance without manual intervention

"Auto-scaling handled the Black Friday traffic spike by adding sixteen additional server instances in under two minutes, keeping response times below 200 milliseconds throughout."

"load balancer"

A component that distributes incoming network traffic across multiple servers to ensure no single server becomes overwhelmed, improving availability and performance

"When the primary load balancer detected that one of the backend servers was returning errors, it automatically removed it from the pool and redistributed traffic to the healthy instances."

"container orchestration"

The automated management of containerised applications across multiple hosts, handling deployment, scaling, networking, and health monitoring — most commonly using Kubernetes

"Container orchestration with Kubernetes allowed the platform team to deploy 40 microservices across three availability zones with a single command, replacing weeks of manual server configuration."

"bandwidth"

The maximum data transfer rate of a network connection, typically measured in megabits per second (Mbps) or gigabits per second (Gbps), affecting how quickly data can be transmitted

"Streaming 4K video to two million concurrent users required the engineering team to renegotiate bandwidth capacity with their CDN provider six months ahead of the product launch."

"latency"

The delay between sending a request and receiving a response, typically measured in milliseconds, which directly affects user experience in real-time applications

"The team moved their database to a region closer to their primary user base, reducing API latency from an average of 180 ms to under 40 ms for 90% of requests."

"redundancy"

The duplication of critical system components so that if one fails, another takes over automatically, ensuring continuous availability and preventing data loss

"The architecture review identified that the payment database had no redundancy — a single server failure would cause complete data loss and a service outage lasting several hours."

"cold start"

The initialisation delay that occurs when a serverless function or container is executed after a period of inactivity, as the runtime must be loaded before the request can be processed

"Cold start latency of up to three seconds was causing poor user experience in the mobile app, prompting the team to use provisioned concurrency to keep functions pre-warmed."

"attack surface"

The total number of different points — APIs, user inputs, open ports, third-party libraries — where an unauthorised user could potentially enter or extract data from a system

"The security audit revealed that years of rapid feature development had dramatically expanded the attack surface, with 47 API endpoints accepting unauthenticated requests."

"zero-day vulnerability"

A software security flaw that is unknown to the software vendor and has no available patch, making it particularly dangerous because defenders cannot protect against a threat they do not know about

"The security team activated their incident response plan immediately after news broke about a zero-day vulnerability in the open-source logging library their entire backend depended on."

"penetration testing"

An authorised simulated cyberattack on a computer system performed by security professionals to identify and report exploitable vulnerabilities before malicious actors find them

"Quarterly penetration testing by an external firm discovered a stored cross-site scripting vulnerability in the user profile page that the internal security scan had missed for six months."

"man-in-the-middle attack"

A cyberattack where an attacker secretly intercepts and potentially alters communication between two parties who believe they are communicating directly with each other

"The developer discovered that mobile app users on public Wi-Fi were vulnerable to a man-in-the-middle attack because the app was not enforcing certificate pinning for its API calls."

"least privilege"

A security principle that grants users, processes, and systems only the minimum permissions required to perform their function, limiting the damage a compromised account can cause

"Applying least privilege to the deployment pipeline meant that even if the CI system was compromised, an attacker could only deploy code — they could not read production secrets or modify IAM policies."

"rate limiting"

A technique that restricts how many requests a user or system can make to an API within a given time window, protecting services from brute-force attacks and denial-of-service attempts

"Adding rate limiting to the login endpoint stopped a credential-stuffing attack that had been attempting 50,000 password guesses per minute against user accounts."

"encryption at rest"

The practice of encrypting stored data — on disk, in databases, or in backups — so that it cannot be read even if the physical storage medium is stolen or accessed without authorisation

"Enabling encryption at rest for the user database was a compliance requirement under GDPR, ensuring that a stolen hard drive could not expose customer personal information."

"social engineering"

A psychological manipulation technique used by attackers to trick people into revealing confidential information or taking actions that compromise security, rather than exploiting technical vulnerabilities

"The most damaging breach the company suffered came not from a technical exploit but from a social engineering attack where an employee was convinced to hand over their credentials over the phone."

"data pipeline"

A series of automated processes that ingest, transform, and deliver data from one or more sources to a destination — such as a data warehouse or analytics dashboard — in a reliable and repeatable way

"A failure in the ETL data pipeline at 3 am caused the morning's sales dashboard to display figures from 24 hours earlier, leading the CEO to make a major inventory decision based on stale data."

"overfitting"

A modelling problem where a machine learning model learns the training data too well — including its noise — and therefore performs poorly on new, unseen data

"The fraud detection model showed 99% accuracy in testing but only 62% in production, a classic sign of overfitting to the specific patterns in the historical training dataset."

"ground truth"

A verified, accurate dataset used to train, validate, or evaluate a machine learning model, representing the correct answers the model is expected to learn

"Building a reliable ground truth for the medical imaging model required three specialist radiologists to independently label 10,000 scans and reach consensus on disputed cases."

"feature engineering"

The process of selecting, transforming, and creating input variables from raw data to improve the performance of a machine learning model

"The biggest accuracy gain did not come from a more complex model but from feature engineering — adding a "days since last purchase" variable improved churn prediction by 18 percentage points."

"vector embedding"

A numerical representation of data — text, images, audio — as a dense vector in a high-dimensional space, where semantically similar items are positioned close together

"The search engine was rebuilt around vector embeddings, replacing keyword matching with semantic similarity — users searching "affordable holiday" now found results for "cheap vacation" automatically."

"hallucination"

A phenomenon in large language models where the model generates confident-sounding but factually incorrect or entirely fabricated information not grounded in its training data

"The legal team rejected deploying the AI assistant for case research after it produced a hallucination — citing a court case with a real-sounding citation that did not exist."

"inference"

The process of using a trained machine learning model to generate predictions or outputs from new input data in a production environment, as distinct from the training phase

"Running inference on millions of images per day required the team to optimise the model for speed, reducing inference time from 480 milliseconds to 12 milliseconds per image."

"data lineage"

The ability to track the origin, movement, and transformation of data across an organisation's systems, essential for debugging pipelines, ensuring compliance, and building trust in analytics

"When the quarterly revenue figure looked wrong, data lineage tooling traced the discrepancy back to a currency conversion that had applied the wrong exchange rate in a transformation step three stages upstream."

"MVP"

Minimum Viable Product — the simplest version of a product that delivers enough value to attract early users and generate the feedback needed to validate or refute core assumptions

"Rather than building all twelve planned features, the team launched an MVP with just the core booking flow, acquiring their first 200 paying customers within two weeks."

"user story"

A short, plain-language description of a software feature written from the perspective of an end user, following the format "As a [user type], I want [goal] so that [reason]"

"The product manager wrote a user story — "As a first-time visitor, I want to see pricing before signing up, so I can decide if the service is affordable" — which resolved a long-running debate about where to display the pricing page."

"A/B test"

A controlled experiment that exposes different groups of users to two or more variations of a product feature simultaneously, then measures which variation better achieves a defined goal

"An A/B test showed that changing the checkout button from green to orange increased completed purchases by 11%, a result that was statistically significant after two weeks of data collection."

"churn rate"

The percentage of users or customers who stop using a product or cancel their subscription within a given time period, a key indicator of product health and customer satisfaction

"A rising churn rate in the second month after sign-up revealed that users valued the onboarding experience but did not find enough ongoing value to justify renewing their subscription."

"pain point"

A specific problem or frustration that users experience when trying to accomplish a goal, which a product feature or improvement aims to alleviate

"Customer interviews revealed that the biggest pain point was not the product itself but the manual data export process users had to perform every Monday morning to update their spreadsheets."

"north star metric"

A single key measurement that best captures the core value a product delivers to its users, used to align the entire organisation around a shared definition of success

"After months of tracking dozens of metrics, the growth team defined "weekly active editors" as their north star metric — the one number that, if it grew, meant the product was genuinely succeeding."

"affordance"

A design quality or visual cue that suggests to a user how an element should be interacted with — a button that looks pressable, a handle that invites grabbing, or a link that signals it is clickable

"Usability testing revealed that removing the border from form fields destroyed their affordance — participants no longer recognised the blank areas as interactive inputs and skipped them entirely."

"technical debt"

The accumulated cost of shortcuts, deferred refactoring, and quick fixes in a codebase that slows future development and increases the risk of bugs if left unaddressed

"Three years of prioritising new features over maintainability had created so much technical debt that adding a simple notification preference took two weeks instead of two hours."

"action item"

A specific, clearly defined task assigned to a named person with a due date, typically documented at the end of a meeting to ensure accountability and follow-through

"The incident post-mortem ended with five action items — each assigned to a specific engineer with a two-week deadline — to prevent the same class of failure from recurring."

"blockers"

Issues, dependencies, or problems that are preventing a person or team from making progress on their work, typically discussed at daily standups and escalated if not resolved quickly

"The developer raised two blockers at the standup: the design file had not been shared yet, and the staging API was returning 500 errors for all authenticated requests."

"stakeholders"

Individuals, teams, or organisations who have an interest in or are affected by a project, including those who fund it, use it, or are responsible for delivering it

"Before writing the technical specification, the architect listed all stakeholders — the engineering team, product management, the legal team, and three enterprise customers — and scheduled interviews to gather their requirements."

"bandwidth"

In a professional context, a person's or team's available capacity to take on additional work, distinct from its technical meaning of network data throughput

"The product manager asked the engineering lead if the team had bandwidth to take on the new integration project, or whether it should be pushed to next quarter to avoid overloading the sprint."

"deep dive"

A thorough, detailed examination of a specific topic, problem, or codebase component — typically in a dedicated meeting, document, or discussion — going beyond surface-level understanding

"After three production incidents in one month, the CTO requested a deep dive into the caching layer to understand why it was invalidating entries prematurely under specific load conditions."

"alignment"

The state where all relevant teams, stakeholders, and decision-makers share the same understanding of a goal, plan, or decision, reducing miscommunication and conflicting work

"The quarterly planning session ended without alignment on the priority between the performance project and the mobile rewrite, requiring a follow-up meeting with the VP of Engineering to make a final call."

"post-mortem"

A blameless meeting or written analysis conducted after an incident or project to understand what happened, identify root causes, and agree on actions to prevent a recurrence

"The post-mortem for the four-hour outage followed a blameless format — the goal was to improve the system, not to assign fault — and produced six actionable improvements to the deployment process."

"low-hanging fruit"

Tasks, improvements, or opportunities that are easy to achieve relative to their benefit, making them a natural starting point before tackling more complex challenges

"The performance audit identified several low-hanging fruit — compressing images, enabling caching headers, and removing an unused analytics script — that improved page load time by 40% with less than a day of work."