What is Backward Compatibility?

Legal Definition
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 abbreviated to BC, or called downward compatibility. Modifying a system in a way that does not allow backward compatibility is sometimes called "breaking" backward compatibility. A complementary concept is forward compatibility, which is a design philosophy, usually based on open standards, that strives for methods that will continue to work with newer and future products. Design that is forward-compatible usually has a roadmap for compatibility with future standards and products.
-- Wikipedia
Legal Definition
When newer software can work with the older version. AKA downward compatibility. Refer to upward compatibility.