PLUG-IN for the Macintosh operating system, that gives the operating system new capabilities.

A part of Macintosh system software that adds specific functions to the computer. An example would be the File Sharing extension that allows networking capabilities to be added to the Mac.

a component which adds functionality to the Mac OS. You can disable extensions wholesale by holding down the Shift key while restarting the Macintosh, or individually via the Extensions Manager. See System Folder, Control Panel, Extensions (Disabled), Extensions folder, device driver.

a startup program that runs when you start the Mac and then enhances its function.

On the Apple Macintosh operating system prior to Mac OS X, extensions were small pieces of code that extended the system's functionality. They were run initially at start-up time, and operated by a variety of mechanisms, including trap patching and other code modifying techniques. Extensions generally filled the same role as DOS's terminate and stay resident programs, or Unix's daemons, although they did have additional functionality to modify existing OS behaviour the other two did not.