When is reference for a definition needed and not needed?
You need to make crystal clear what is your contribution and what isn't. In general, all definitions that are not your contribution need the proper reference.

In practice that could get cumbersome, so you end up not citing what is considered common, basic knowledge. However, that line is somewhat blurry and one needs experience to do it right.

You can check good papers in your field to guess what is/isn't common knowledge. Another interesting tactic, that I often use, is to say: 


  "Since we are using XXX, we will briefly review the involved concepts [X]".


That way it is clear that they are not your contribution and that reference [x] can be consulted for further details, without too many references and unnecessary text...