A double-headed horizontal arrow symbol representing logical equivalence or bidirectional implication in mathematical logic.
Examples
Logical equivalence
A \hArr B \text{ means 'A if and only if B'}
Mathematical equivalence
x^2 = 4 \hArr x = \pm 2
Set equivalence
|A| = |B| \hArr \text{There exists a bijection } f: A \to B