Transposition table
A cache of already-analysed game positions, so a search doesn't recompute the same position just because it arrived by a different move order.
In chess, 1.e4 e5 2.Nf3 and 1.Nf3 e5 2.e4 reach the same position — a transposition. Without memory the search analyses it twice, and deeper in the tree the duplicates run into the millions. The table is a large hash table keyed on a position hash, usually Zobrist, storing the search depth, the score, whether that score is exact or only a bound, and the best move.
That last field matters as much as the score: under iterative deepening the previous iteration's best move is tried first, which makes alpha-beta considerably more efficient. The table is never big enough, so engines have replacement policies deciding which entries may be overwritten — a classic cache trade-off in a new outfit.