Backward compatibility is a property of a system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially in telecommunications and computing. Backward compatibility is sometimes also called downward compatibility.
A complementary concept is forward compatibility. A design that is forward-compatible usually has a roadmap for compatibility with future standards and products.
A good example is the Sony PlayStation 2 (PS2) which was backward compatible with games for its predecessor PlayStation (PSX or PSOne). While the PS2's selection of launch titles was small, sales of the console were nonetheless strong in 2000-2001 thanks to the large library of games for the preceding PSX/PSOne. This bought time for the PS2 to grow a large installed base and developers to release more quality PS2 games for the crucial 2001 holiday season.
A good example is the Sony PlayStation 3, as the first PS3 iteration was expensive to manufacture in part due to including the Emotion Engine from the preceding PS2 in order to run PS2 games, since the PS3 architecture was completely different from the PS2. Subsequent PS3 hardware revisions have eliminated the Emotion Engine as it saved production costs while removing the ability to run PS2 titles, as Sony found out that backwards compatibility was not a major selling point for the PS3 in 2006-2009, in contrast to the PS2 in 2000-2001. The PS3's chief competitor, the Microsoft Xbox 360, took a different approach to backwards compatibility by using software emulation in order to run games from the first Xbox, rather than including legacy hardware from the original Xbox which was quite different than the Xbox 360, however Microsoft stopped releasing emulation profiles after 2007.
Dennie van Tassel has remarked: "I've finally learned what 'upward compatible' means. It means we get to keep all our old mistakes."
Full backward compatibility is particularly important in computer instruction set architectures, one of the most successful being the x86 family of . Their full backward compatibility spans back to the 16-bit Intel 8086/8088 processors introduced in 1978. (The 8086/8088 in turn were designed with easy machine-translatability of programs written for its predecessor in mind, although they were not instruction-set compatible with the 8-bit Intel 8080 processor as of 1974. The Zilog Z80, however, was fully backwards compatible with the Intel 8080.) Fully backward compatible processors can process the same executable as their predecessors, allowing the use of a newer processor without having to acquire new applications or . Similarly, the success of the Wi-Fi digital communication standard is attributed to its broad forward and backward compatibility; it became more popular than other standards that were not backward compatible.