I used lichess to analyse some of my games (lichess.org -> tools -> import game). It puts my bad moves (and those of my opponent) in one of three categories; "inaccuracy", "mistake" and "blunder". I guessed from the names (and the positions) that an inaccuracy is better than a blunder, but I wondered what the definitions were.
Blunder: 300 centipawns
Mistake: 100 centipawns
Inaccuracy: 50 centipawns
This is sourced from http://en.lichess.org/qa/75/how-are-these-determined-blunder-mistake-and-inaccuracy
It is the first thing that appears when you google "lichess inaccuracy mistake blunder".