\LeftrightarrowLogical equivalence or biconditional, often used in mathematical logic to denote that two statements are logically equivalent.
A is logically equivalent to B
A \Leftrightarrow Bx equals 2 if and only if x squared equals 4
x = 2 \Leftrightarrow x^2 = 4Predicate P(n) is equivalent to predicate Q(n)
P(n) \Leftrightarrow Q(n)