Matrix (mathematics)

In mathematics, a matrix (plural matrices) is a rectangular array or table of number, symbol, or expression, arranged in rows and columns, which is used to represent mathematical object or a property of such an object.
— Wikipedia

For example, $\left[\begin{array}{ccc}1& 9& -13\\ 20& 5& -6\end{array}\right]$ is a matrix with two rows and three columns. This is often referred to as a “two by three matrix”, a ”$2\times 3$ matrix”, or a matrix of dimension $2\times 3$.

Links to array and expression are from computer science, but better to check corresponding definitions in mathematics.