Inverse Matrix
The inverse of a square matrix is another matrix that, when multiplied by the original matrix, yields the identity matrix. A matrix only has an inverse if its determinant is not equal to zero (a non-singular matrix).
Example
For a $2 \times 2$ matrix $A = \begin{pmatrix} a & b \\ c & d \end{pmatrix}$ with determinant $|A| \neq 0$, the inverse is: $$A^{-1} = \frac{1}{|A|} \begin{pmatrix} d & -b \\ -c & a \end{pmatrix}$$