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)