A relational symbol denoting geometric congruence or isomorphic structures in algebra and geometry.
Examples
Congruent triangles in geometry
\triangle ABC \cong \triangle DEF
Modular arithmetic congruence
15 \cong 3 \pmod{12}
Isomorphic groups in abstract algebra
\mathbb{Z}_4 \cong \mathbb{Z}_2 \times \mathbb{Z}_2