Matrix
A matrix is a rectangular array or grid of numbers, symbols, or expressions arranged in rows and columns. Matrices are widely used to solve systems of linear equations and to represent geometric transformations.
Example
A $2 \times 2$ matrix $A$ can be written as: $$A = \begin{pmatrix} 1 & 2 \\ 3 & 4 \end{pmatrix}$$ where $1$ and $2$ form the first row, and $1$ and $3$ form the first column.