1. The problem asks about the determinant of a matrix $M$ when $M$ is not a square matrix. 2. The determinant, denoted as $\det(M)$, is defined only for square matrices, which means matrices with the same number of rows and columns (e.g., $n \times n$). 3. If $M$ is not square, for example, if it is $m \times n$ with $m \neq n$, then $\det(M)$ is not defined. 4. This is because the determinant is a scalar value that summarizes certain properties of square matrices, such as invertibility and volume scaling, which do not apply to non-square matrices. 5. Therefore, if $M$ is not square, $\det(M)$ does not exist or is undefined. Final answer: The determinant $\det(M)$ is undefined if $M$ is not a square matrix.