A bidirectional arrow symbol representing logical equivalence, biconditional statements, or bijective mappings between sets.
Examples
Logical equivalence
A \longleftrightarrow B \text{ means 'A if and only if B'}
Algebraic equivalence
x^2 = 4 \longleftrightarrow x = \pm 2
Set bijection
f: X \longleftrightarrow Y \text{ denotes a bijective function}