A matrix is a list of numbers arranged in a rectangle with rows and columns. In programming, it is also known as a 2D array. Use this tag if your challenge is about manipulating matrices.
In mathematics matrices are often used to represent linear transforms from one vector space to another. If your challenge involves such matrices consider adding the additional tag of linear-algebra.
If your challenge has to do with matrices that can only contain 1s and 0s, consider adding the tag binary-matrix.