Source : Free On-Line Dictionary of Computing

alpha/beta pruning
     
         An optimisation of the {minimax}
        {algorithm} for choosing the next move in a two-player game.
        The position after each move is assigned a value.  The larger
        this value, the better the position is for me.  Thus, I will
        choose moves with maximum value and you will choose moves with
        minimum value (for me).
     
        If it is my move and I have already found one move M with
        value alpha then I am only interested in other moves with
        value greater than alpha.  I now consider another of my
        possible moves, M', to which you could reply with a move with
        value beta.  I know that you would only make a different reply
        if it had a value less than beta.  If beta is already less
        than alpha then M' is definitely worth less than M so I can
        reject it without considering any other replies you might
        make.
     
        The same reasoning applies when considering my replies to your
        reply.  An alpha cutoff is when your reply gives a lower value
        than the current maximum (alpha) and a beta cutoff is when my
        reply to your reply gives a higher value than the current
        minimum value of your reply (beta).
     
        In short, if you've found one possible move, you need not
        consider another move which your opponent can force to be
        worse than the first one.
     
        (1997-05-05)