Transposition table
:
The transpostion table is a hashing scheme to detect positions in different branches of the search tree that are identical. If a search arrives at a position that has been reached before and if the value obtained can be used, the position does not have to be searched again.
Chess Tree Search
MiniMax and NegaMax
Alpha-Beta search
Aspiration search
Transposition table 
A transposition table is a safe optimization that can save much time. The only danger is that mistakes can be made with respect to draw by repetition of moves because two positions will not share the same move history.