A matrix (or matrices in plural) is a set of values organized into columns and rows and enclosed in brackets:

This matrix, Matrix A, has two rows and two columns, so is called a “2 by 2”, or 2×2 matrix (where dimensions are described as rows x columns.)

Each entry or element in a matrix has an address describing its location. This location is written as the lower case matrix letter followed by the element’s row number and column number in subscript. For example, in the matrix above, the element b is in row 1, column 2 of matrix A, so it’s location is described as a₁₂.