- forced checkmate
- more than adequate material compensation
- clearing the way for a P's promotion
- subsequent capture of the opponent's Q that results a positional or material gain
Bent Larsen wrote that giving up the Q for a R and two minor pieces is sometimes called a "queen sacrifice", but since a Rook plus two minor pieces is more valuable than the Q, he said it should not be considered a sacrifice.
In this game white exchanges his Q for adequate material plus he got an overwhelming attack. We still like to call it a Q-sacrifice though, even if, technically when we apply Larsen's definition, it wasn't.