Source : Free On-Line Dictionary of Computing

Busy Beaver
     
         (BB) One of a series of sets of {Turing Machine}
        programs.  The BBs in the Nth set are programs of N states
        that produce a larger finite number of ones on an initially
        blank tape than any other program of N states.  There is no
        program that, given input N, can deduce the productivity
        (number of ones output) of the BB of size N.
     
        The productivity of the BB of size 1 is 1.  Some work has been
        done to figure out productivities of bigger Busy Beavers - the
        7th is in the thousands.
     
        (1994-10-24)