Beta (β) is the upper bound of a score for the node. If the node value exceeds or equals beta, it means that the opponent will avoid this node, since his guaranteed score (Alpha of the parent node) is already greater. Thus, Beta is the best-score the opponent (min-player) could archive so far - in a Negamax framework from max-players point of view, where beta is the negated value of alpha from the parent node. If a beta-cutoff occurs on the upper bound of a Cut-Node given by Beta, the value returned is a lower bound.
Beta
Contributors: GerdIsenberg.