In mathematics, a **domino** is a polyomino of order 2, that is, a polygon in the plane made of two equal-sized squares connected edge-to-edge.^{[1]} When rotations and reflections are not considered to be distinct shapes, there is only one *free* domino.

Since it has reflection symmetry, it is also the only *one-sided* domino (with reflections considered distinct). When rotations are also considered distinct, there are two *fixed* dominoes: The second one can be created by rotating the one above by 90°.^{[2]}^{[3]}

In a wider sense, the term *domino* is sometimes understood to mean a tile of any shape.^{[4]}

Main article: Domino tiling |

Dominos can tile the plane in a countably infinite number of ways. The number of tilings of a 2×*n* rectangle with dominoes is , the *n*th Fibonacci number.^{[5]}

Domino tilings figure in several celebrated problems, including the Aztec diamond problem in which large diamond-shaped regions have a number of tilings equal to a power of two,^{[6]} with most tilings appearing random within a central circular region and having a more regular structure outside of this "arctic circle", and the mutilated chessboard problem, in which removing two opposite corners from a chessboard makes it impossible to tile with dominoes.^{[7]}