Matrix Definition

#Math
An $\displaystyle n\times m$ matrix has $\displaystyle n$ rows and $\displaystyle m$ columns
$$
\begin{bmatrix}
a_{11} & \cdots & a_{1m} \\\vdots & \ddots & \vdots \\a_{n1} & \cdots & a_{nm}
\end{bmatrix}
$$

A system of linear equation's coefficient matrix would have $\displaystyle n$ equations and $\displaystyle m$ variables