it has become customary to speak of problems that are solvable, not only in principle but also in practice, as tractable; problems that may be solvable in principle but are not solvable in practice are then called intractable.
the problem of determining membership of strings in a given language is tractable if and only if .
(taken from Martin Davis, Ron Sigal, Elaine J. Weyuker, 1994 chapter 15 polynomial-time computability)