# How to Say Side Effect in English: Tech Audio & Practice (2026)

> An unintended change in program state caused by executing code, where a function modifies something outside of its primary purpose, such as modifying a global variable or changing external data.

- **Term:** side effect
- **Part of speech:** noun
- **Pronunciation:** /ˈsaɪd ɪˈfekt/
- **English level (CEFR):** B2
- **Register:** Neutral, Both
- **Region:** US and UK
- **Topic:** [Code and Git terms](https://app.speaktechenglish.com/learn/topics/software-development-terms.md)
- **Lesson:** https://app.speaktechenglish.com/learn/side-effect
- **Last updated:** 2026-10-27

## Meaning

An unintended change in program state caused by executing code, where a function modifies something outside of its primary purpose, such as modifying a global variable or changing external data.

In software development, a side effect occurs when a function or method does something beyond returning a value to the caller. For example, a function that fetches user data might also log the request to a database, update a cache, or modify global state. While some side effects are necessary and intentional, developers generally try to minimize them because they make code harder to test, debug, and predict. When a function has no side effects (pure function), it becomes easier to understand, test, and reuse in different contexts. Managing side effects is a core concern in code reviews and architecture discussions, especially when designing systems that must be reliable and maintainable.

## When you hear it

During code review, a senior developer points out that the new function has multiple side effects that make it difficult to test.

## Examples

- **in a code review:** "This function modifies the global cache as a side effect, which makes it harder to unit test."
- **in a standup:** "We discovered that the authentication function had an unexpected side effect of clearing user sessions."
- **in a team meeting:** "To improve maintainability, we should refactor these components to minimize side effects."

## What native speakers say

- "This function has too many side effects"
- "We need to eliminate side effects from this module"
- "Be careful about side effects when refactoring"

## Common mistakes

- ✗ "The side effect of the code is that it works faster now." → ✓ "The side effect of the code is that it modifies the global state unexpectedly." (Side effect means an unintended state change, not a result or benefit. Use 'benefit' or 'result' for positive outcomes.)
- ✗ "This function is side effect." → ✓ "This function has side effects or This is a pure function without side effects." (Side effect is a noun phrase, not an adjective. You cannot describe something as side effect; you describe what has side effects.)
- ✗ "Can you side effect this code?" → ✓ "Can you remove the side effects from this code?" (Side effect is not typically used as a verb. Use phrases like remove side effects, minimize side effects, or manage side effects.)

## Be careful

Non-tech speakers may confuse it with the medical meaning (adverse reaction to medicine). In tech, it refers to when code changes something beyond its intended output, not about negative consequences.

## Word forms

| Part of speech | Word | Example |
| --- | --- | --- |
| noun | side effect | The function has a side effect that modifies the database. |
| noun phrase | side effects (plural) | We need to eliminate the side effects in this module. |
| adjective | side-effect-free or pure | Pure functions are side-effect-free and easier to test. |


## Collocations

- **has side effects**: Most common collocation; describes when code causes unintended state changes
- **minimize side effects**: Describes the goal of reducing unwanted state changes in code
- **unintended side effect**: Emphasizes that the state change was not planned or expected
- **side effects occur**: Describes when side effects happen during execution
- **remove side effects**: Describes refactoring code to eliminate state changes

## Synonyms

- **unintended consequence**: More general term; side effect is more specific to code behavior
- **state mutation**: More technical; refers specifically to changing program state
- **external change**: Broader term; side effect is more precise in technical contexts

## Opposites

- pure function
- no side effects

## Practice

This lesson has 4 practice exercises with instant feedback on the page: https://app.speaktechenglish.com/learn/side-effect

## FAQ

### What does side effect mean in software engineering?

In software engineering, a side effect is any change in program state that occurs as a result of executing code, beyond the functions primary return value. This includes modifying global variables, changing files on disk, updating a database, writing to the console, or altering class state. Side effects make code less predictable and harder to test, which is why developers try to minimize them.

### How do you use side effect in a sentence at work?

You typically use it when describing code behavior in reviews or meetings. Examples: 'This function has side effects that make it difficult to test,' 'We need to refactor this module to remove side effects,' or 'The caching mechanism has an unintended side effect of storing stale data.' You can also use it in phrases like side-effect-free or side effects occur.

### Is side effect formal or informal?

Side effect is neutral in formality. It is equally appropriate in casual standups, formal code reviews, architecture documents, and job interviews. It is standard technical terminology used by all levels of engineers.

### What is the difference between side effects and errors?

Side effects are not errors. They are unintended state changes that may or may not cause problems. An error is when code fails to execute or produces an exception. However, side effects can make errors harder to diagnose because they change system state in unexpected ways.

### Why do developers want to eliminate side effects?

Developers minimize side effects because they make code harder to test, debug, reason about, and maintain. Functions without side effects (pure functions) always produce the same output for the same input, making them predictable and reliable. This is especially important in large systems where unexpected state changes can cause subtle bugs.


## Related

- [pure function](https://app.speaktechenglish.com/learn/pure-function.md)
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