Definition. A modular extension package for TYPO3 that adds functionality, content types, plugins or configuration. Extensions are installed via Composer, register their building blocks through ext_emconf/ext_localconf and can cover both frontend and backend aspects. Custom site packages are technically extensions as well.

Why it matters. The extension model is one of the reasons TYPO3 can meet individual requirements without bending the core. It is also the point where maintainability and upgradability are decided.

Related. TYPO3, Open Source CMS, Enterprise CMS, Structured Content Modelling