non-deterministic Turing machine
may have a set of rules that prescribes more than one action for a given situation; state and tape symbol no longer uniquely specify things; rather, many different actions may apply for the same combination of state and symbol
unambiguous Turing machine
special kind of non-deterministic Turing machine, which, in some sense, is similar to a deterministic Turing machine; for each input, there is exactly one computation possible
linear bounded automaton
Turing machine, in which computation is restricted to the portion of the tape containing the input