How long it takes to solve a problem. A problem has an inherent complexity (minimum time needed to solve it); any algorithm to solve the problem will have a higher complexity (take at least that long).
A tractable problem is one that can be solved in a reasonable amount of time; usually the distinction between tractable and intractable is drawn at the boundary between problems that can be solved in an amount of time that is polynomial; those that require exponential time are regarded as intractable.