The Laplacian spectral radius of a finite graph is defined as the largest value of its Laplacian spectrum, i.e., the largest eigenvalue of the Laplacian matrix (Lin et al. 2023) or largest root of the Laplacian polynomial.
The ratio of the Laplacian spectral radius to algebraic connectivity is known as the Laplacian spectral ratio.
More things to try:
Weisstein, Eric W. "Laplacian Spectral Radius." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/LaplacianSpectralRadius.html