XML dictionary file created for scripts written in the AppleScript language; contains a dictionary of terms, which includes classes (nouns), properties (adjectives), commands (verbs), and parameters (adverbs); stores the terminology used to communicate with the underlying program being controlled by the script.

AppleScript dictionaries store terminology in a format similar to human language. Because of this, script authors can write scripts using developer-defined terms that are easy to understand. SDEF files can be created and edited using AppleScript Editor, which is included with Mac OS X. AppleScript is a scripting language built into Mac OS X that is used to automate repetitive tasks, create shortcuts, and develop custom applications. SDEF files can be used for incorporating custom instructions into AppleScript. NOTE: AppleScript Editor was previously called "Script Editor" in Mac OS X 10.5 (Leopard) and earlier. The program was renamed to "AppleScript Editor" when Mac OS X 10.6 (Snow Leopard) released.