Source : Free On-Line Dictionary of Computing

Nondeterministic Turing Machine
     
         A normal (deterministic) {Turing Machine} that
        has a "guessing head" - a write-only head that writes a guess
        at a solution on the tape first, based on some arbitrary
        internal {algorithm}.  The regular {Turing Machine} then runs
        and returns "yes" or "no" to indicate whether the solution is
        correct.
     
        A {nondeterministic Turing Machine} can solve
        {nondeterministic polynomial time} computational {decision
        problems} in a number of steps that is a {polynomial} function
        of the size of the input
     
        (1995-04-27)