Hoping to tide you over until a better answer comes along... Generally when complex programs (like an entire programming language) are made, there is usually a

Hoping to tide you over until a better answer comes along... Generally when complex programs (like an entire programming language) are made, there is usually a "specification" of its behaviors. This way, you can communicate to people, hey, my program does ABC and does NOT do XYZ. Also, it helps you delineate between bugs and what not. My program does AB DEF. Is that a bug? Or a feature? What does the specification say? Not all programs have a specification. It's a bit formal and unnecessary. Except in a specific scenario where you have multiple/competing implementations of the same programming language. Now that there are multiple players, we need to know exactly which behaviors are per the spec and which behaviors are per the implementation. All coffee makers must make ABC XYZ. This is the spec. Sony CoffeeMate makes ABC DEF XYZ. It matches the spec and throws in some extra features. Samsung Coffee Xtreme makes ABC PQR XY. This also throws in extra features but is actually missing one from the spec. TL;DR it helps a language evolve independently of its implementations (come the day that a language != its implementation)