noun
In computing, refers to software that adds new features to an existing program.
このブラウザの機能拡張を使うと、広告をブロックできます。
Using this browser extension, you can block ads.
プラグイン is a more specific term often used for software components that integrate into a host application, while 機能拡張 is a broader term for any feature extension.
Compound of 機能 (function) and 拡張 (expansion).