Actually Crash, any definition from any source is an ok place to start. When two people are attempting to communicate they may find they aren't useing the same definition of words. Then they have to clarify what they mean.
With luck some dictionary definitions will make that easy.
We have had extensive discussions of the definition of "atheist". It doesn't matter what the word means in any dictionary. It is necessary to figure out what the individual themselves thinks and believes. If that happens to fit exactly some dictionary definition then great. If not they need to come up with another version or decide that they are not an "atheist" or whatever the word is.
We have, I think, had people use the term "positive atheist" which, IIRC, means that the do actually have a belief about gods -- that is that there isn't one. I suppose we will have to invent a half dozen terms to be able to get just the right one for each individual.