The letters in a filename that appear to the right of the dot ('.jpg', '.gif', '.eps', '.wmf', '.zip', '.hqx', etc.). Extensions indicate what kind of file you have; for instance, a JPEG's extension is '.jpg'. Also referred to as a 'suffix.'
In Windows, the extension must match the file's format; changing it may alter the format or render it unreadable. In Macintosh, however, the extension is simply part of the filename and will not affect the file it is attached to.