An aspiration window is a heuristic used in pair with alpha-beta pruning in order to reduce search time for combinatorial games by supplying a window (or range) around an estimated score guess.
Use of an aspiration window allows alpha-beta search to compete in the terms of efficiency against other pruning algorithms.
[1] Alpha-beta pruning achieves its performance by using cutoffs from its original range.
[2][example needed] However, due to search instability, the score may not always be in the window range.
[2] Despite this, popular engines such as Stockfish still use aspiration windows.