It's the standard sign for congruency in modular arithmetic, if that's what setting you are in. := is "defined as" in most math software.

Depending on the context it's either "congruent to" or "identically equal to". It's mainly used as "congruent to" when dealing with modular arithmetic. -Tom

This is right...well, whenever I've seen it used recently, but I have seen it used in other terms too. But this is the one I see most commonly in physics and calc.