MemberQ[list, form] returns True if an element of list matches form, and False otherwise.
MemberQ[list, form, levelspec] tests all parts of list specified by levelspec.
form can be a pattern.
Example: MemberQ[x^2, y^2, x^_] .
The first argument of MemberQ can have any head, not necessarily List.
MemberQ[list, form] immediately tests whether any expression in list matches form; Element[x, dom] asserts that x is an element of the symbolic domain dom.
See The Mathematica Book: Section 1.8.5 and Section 2.3.5.
See also: FreeQ, Element, Count, Cases, IntervalMemberQ.