Able to share data or commands with older versions of itself, or sometimes other older systems, particularly systems it intends to supplant. Sometimes backward compatibility is limited to being able to read old data but does not extend to being able to write data in a format that can be read by old versions.
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.
Compare: forward compatible.