In chess, a sacrifice is a move that gives up a piece with the objective of gaining tactical or positional compensation in other forms.
Because players usually try to hold on to their own pieces, offering a sacrifice can come as an unpleasant surprise to one's opponent, putting them off balance and causing them to waste precious time trying to calculate whether the sacrifice is sound or not, and whether to accept it.
[2] Rudolf Spielmann proposed a division between sham and real sacrifices: In compensation for a real sacrifice, the player receives dynamic, positional, or other non-material advantages which they must capitalize on, or risk losing the game due to the material deficit.
Non-forcing sacrifices, on the other hand, give the opponent a choice.
A common error is to not recognize when a particular sacrifice can be safely declined with no ill-effects.
Re1+!, Aronian resigned, because Black's move forces the reply 25.Rxe1 (or 25.Qf1 Qxf1#), after which White's queen is undefended and therefore lost.
White intends to keep checking on the seventh rank, and if Black ever captures the rook it is stalemate.
Having forced the rook out of a position where it was defending the f-file and into a position where it blocked the king from making any move, the black knight delivers a smothered mate by 23... Nf2#.
[12] An example of this real, strategic/positional sacrifice can occur in Petrov's Defense after 1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 where White elects 4.Nxf7 Kxf7 (diagram).