Plugin
A software add-on that extends the functionality of a platform or design tool — adjacent professional vocabulary.
In practice
A plugin adds a specific function to an existing tool without changing the tool itself, much like an attachment on a kitchen mixer. Designers install plugins to automate repetitive work or bring in outside resources: generating placeholder text, checking color contrast for accessibility, or renaming hundreds of layers at once. Each plugin is also a dependency, so teams usually agree on which ones a shared file may rely on.
Often confused with API
An API lets separate programs talk to each other; a plugin is installed inside one program to extend it, often by using that program's API.
Try this
Find a contrast-checking plugin or web tool and test the text colors in one of your recent designs.