A binary operator symbol showing a box with a diagonal cross inside, commonly used in algebraic topology and ring theory.
Examples
Box tensor product of rings
R \boxtimes S \cong R \otimes_\mathbb{Z} S
External product of cohomology groups
H^p(X) \boxtimes H^q(Y) \to H^{p+q}(X \times Y)
Box product of matrices
A \boxtimes B = (a_{ij}B)_{m \times n}