In computational complexity theory, P is one of the most fundamental complexity classes. It contains all decision problems which can be solved by a deterministic Turing machine using a polynomial amount of computation time, or polynomial time.
P is often taken to be the class of computational problems which are "efficiently solvable" or "tractable", although there are potentially larger classes that are also considered tractable such as RP and BPP. Also, there exist problems in P which are intractable in practical terms; for example, some require at least n^1000000 operations. See even harder problems of complexity classes for further discussion.
Reference:
http://en.wikipedia.org/wiki/P_%28complexity%29
