A scripting addition extends the vocabulary available to scripting languages such as AppleScript without requiring the scripter to target a specific application to handle events.  Unfortunately, scripting additions all live in the same namespace, inevitably resulting in name collisions, and they can't define classes, which means they can't implement the AppleEventObjectModel.

In power user jargon, a scripting addition may be called an OSAX (OSA eXtension), since 'osax' is both the file type of the scripting addition file and the resource type of the code resource contained therein.  The plural is OSAXen.