A relational symbol denoting isomorphism between structures or approximate equality in mathematics and physics.
Examples
Isomorphism between algebraic structures
\mathbb{Z}/2\mathbb{Z} \simeq \{0,1\}
Homotopy equivalence between topological spaces
S^1 \simeq \mathbb{R}^2 \setminus \{0\}
Group isomorphism with explicit mapping
G/H \simeq K \text{ under } \phi: x \mapsto x^2