What does this statement describe? $X$ and $Y$ are matrices.



> 
> The eigenvalues of $X$ in the metric of $Y$.
> 
> 
> 


I've not seen this language used before in this fashion and I don't really know what taking the eigenvalues of a matrix in the metric of another matrix means. 


Could someone decipher this for me?



Eigenvalues of $X$ in the metric of $Y$
It means the $\lambda$ such that $X - \lambda Y$ is not invertible. The usual eigenvalues are those where $Y$ is the identity matrix. 


