# Define unfamiliar terms and abbreviations (EK00009)

import { Aside, Badge } from '@astrojs/starlight/components';

Spelling out the full term the first time you use an abbreviation lets readers who don't recognize it follow along. EkLine flags an abbreviation used before you define it (rule EK00009) so you can introduce the term on first use.

## Why define abbreviations in documentation?

Documentation reaches readers with a range of backgrounds, and an undefined abbreviation forces anyone who doesn't know it to guess or search elsewhere. Defining the term once, on first use, keeps the page self-contained and accessible. After that first definition, the abbreviation alone is clear for the rest of the document.

## What EkLine flags

EK00009 matches an abbreviation that is used without being defined on first use. Common, widely understood abbreviations are exempt.

- ❌ **Incorrect:** Configure the SSO settings.
- ✅ **Correct:** Configure the single sign-on (SSO) settings.
- ❌ **Incorrect:** Enable RBAC for each project.
- ✅ **Correct:** Enable role-based access control (RBAC) for each project.

## How to fix

Spell out the full term on first use and put the abbreviation in parentheses after it. Use the abbreviation on its own for every mention that follows.

<Aside type="note">
Widely recognized abbreviations don't need a definition. When a term is already common for your audience, keep the abbreviation and ignore the rule for that line.
</Aside>

## FAQ

### When should you spell out an abbreviation?

Spell out the full term on first use and put the abbreviation in parentheses after it. Use the abbreviation on its own for every mention that follows.

### Which abbreviations don't need a definition?

Widely recognized abbreviations that are already common for your audience don't need a definition. Keep those and ignore the rule for that line.

## Rule details

| Property | Value |
|----------|-------|
| Rule ID | `EK00009` |
| Category | Consistency |
| Default severity | <Badge text="Warning" variant="caution" /> |
| Enabled by default | No |

## Ignore this rule

To keep an undefined abbreviation on one line, add an inline comment:

```markdown
<!-- ekline-ignore-next-line EK00009 -- defined in the glossary -->
Configure the SSO settings.
```

To disable the rule across your project, add it to the `ignore` list in your [configuration file](/reviewer/configuration/ignoring-rules/#ignore).

## Related rules

- [EK00016](/reviewer/rules/ek00016/) — Avoid periods in acronyms and initialisms
- [EK00002](/reviewer/rules/ek00002/) — Avoid acronyms in titles or headings