Extension

1. When referring to an IBM compatible computer, a file's extension are the three characters after the file name. The file extension is used by the operating system and user to easily identify the type of file and what software program the file should be used with.
   
   Additional information about computer file extensions can be found on our file extension page.
   
   
2. When referring to an Apple Macintosh computer, extensions or system extensions are files that are stored in the extension folder that load into memory when the computer first loads. Extensions were known as an INIT on System 6 and earlier.

Also see: Association, File extension, INIT, System