A bidirectional arrow symbol representing logical equivalence, biconditional statements, or bijective mappings between sets.
Examples
Logical equivalence
A \leftrightarrow B \text{ means 'A if and only if B'}
Bidirectional mapping
f: X \leftrightarrow Y \text{ is a bijective function}
Chemical equilibrium
N_2 + 3H_2 \leftrightarrow 2NH_3