A suffix is something added at the end of a word that conditions its usage or meaning. In computer system file names, a suffix is a convention for having one or more characters appended to a file name (usually separated from the file name with a dot) so that it can be distinguished from other files or grouped together with similar types of files. For example, all files created with a WordPerfect word processor can be associated with a ".wpd" suffix. That way, a user can easily distinguish it as a file created with that word processor from files created with other tools and selectively process a number of similar files.
Different operating systems have different rules about how long the suffix part of a file name can be. DOS, Windows prior to Windows 95, and OS/2 prior to OS/2 Warp limit suffix names to a maximum of three characters. UNIX systems, Windows 95, and Mac systems permit longer suffix names.