| context | $ A \in \mathrm{SquareMatrix}(n,\mathbb C) $ |
| definiendum | $ \sqrt{\lambda} \in \mathrm{SingularVal}(A) $ |
| postulate | $ \lambda \in \mathrm{EigenVal}(A^*A) $ |
Note that $A^*A$ is always Hermitian positive semi-definite matrix.
Singular values make sense for more general operators too.
Wikipedia: Singular value, Matrix norm