The eigenvalue of a Maass form (reviewed)

The eigenvalue $\lambda$ of a Maass newform on GL(2) refers to the eigenvalue of the eigenspace of the Laplace-Baltrami operator in which it lies.

This eigenvalue is a real number that is typically written in the form $\lambda=\frac{1}{4}+R^2$, where $R$ is the spectral parameter.