A system is backward compatible if it is compatible with (e.g. can share data with) earlier versions of itself, or sometimes other earlier systems, particularly systems it intends to supplant.

For example, WordPerfect 6.0 can read WordPerfect 5.1 files, so it is backward compatible. It can be said that Perl is backward compatible with awk, because Perl was (among other things) intended to replace awk, and can, with a converter, run awk programs.

See also: backward combatability.

Compare: forward compatible.