I'd like to point out that there was a different question (I think the most recent incarnation of this question, http://meta.math.stackexchange.com/questions/825/how-should-we-handle-tags-terminology-notation-definition, that was raised on meta two years ago. But as I read through, I thought there was a focus on keeping notation separated from the others (which I agree with - notation is different).
The wiki for terminology says:
Questions on the usage and meaning of words in mathematics, the names for mathematical entities, and other such questions.
The wiki for definition is longer, and says:
Definitions are the core of mathematical precision; they come to answer "what is X" in mathematics. Into this category fit questions regarding equivalence of definitions; clarifications regarding complicated definitions; as well questions with purposed definitions for mathematical notions with requests for improvements or comments.
Perhaps these are clearly different to someone, but when I see something about "the meaning of words in mathematics... and other such questions" compared to looking at "what is 'X' in mathematics", they look about the same to me.