1 Of hardware, the ability of a subsystem (e.g. memory) or an external device (e.g. a terminal) to be substituted for the originally designated equipment. To designate that one manufacturer's hardware can be connected to another manufacturer's hardware, the terms plug-to-plug compatible, or plug-compatible, are used. The ability of new hardware to handle interfaces of previous generations is called backward compatibility.
2 Of software, the ability of a computer to execute directly program code that was compiled, assembled, or written in machine language for another computer. Generally this occurs for successive computers in a given manufacturer's line. Since later computers are usually more capable (i.e. have a larger instruction set and/or more memory), the ability to run the program of a less capable machine is usually called upward compatibility or backward compatibility. See also portable, emulation.
3 Of a new piece of software, the ability to reproduce the behavior of its predecessor, in particular to accept the same input formats.