The extent to which a given wave function, ψ, is entangled is measured by minimizing the norm of ψ minus all possible unentangled functions. This measure is given by the largest eigenvalue of ψ†ψ, considered as an operator. The definition is basis independent.
Cite
@article{arxiv.quant-ph/9701030,
title = {Measure of Entaglement},
author = {L. Schulman and D. Mozyrsky},
journal= {arXiv preprint arXiv:quant-ph/9701030},
year = {2007}
}