The name of a file. All files have names. Different operating systems impose different restrictions on filenames. Most operating systems, for example, prohibit the use of certain characters in a filename and impose a limit on the length of a filename. In addition, many systems, including DOS and UNIX, allow a filename extension that consists of one or more characters following the proper filename. The filename extension usually indicates what type of file it is.
Within a single directory, filenames must be unique. However, two files in different directories may have the same name. Some operating systems, such as UNIX and the Macintosh operating system, allow a file to have more than one name, called an alias. (In Unix, aliases are called links or symbolic links.)
See the Data Formats and Their File Extensions page in the quick reference section of Webopedia for a list of file extensions and the programs they coincide with.