A relational symbol denoting mathematical equivalence or identical equality between expressions, definitions, or congruences.
Examples
Congruence in modular arithmetic
15 \equiv 3 \pmod{6}
Logical equivalence
p \equiv q \text{ means } (p \rightarrow q) \land (q \rightarrow p)
Function identity
f(x) \equiv x^2 + 2x + 1