The minor of any entry of a square matrix is the determinant of the matrix that remains when you delete the row and column containing that entry. For example, the minor of the number 4 in the determinant   is the sub-determinant   .

Here's an example that shows all the minors and cofactors of a typical 3x3 matrix.

The cofactor of any entry of a square matrix is its "signed minor" - its minor with a sign attached. Choose the sign corresponding to the element's position in the matrix according to a "checkerboard pattern' like this one:

For example, the the sign associated with the position of the number 4 in the determinant is –, so the cofactor of the number 4 is the signed minor.