# Definition of curvature

Kay (Fundamentals of Statistical Signal Processing) defines the curvature of a log-likelihood function to be the "negative of the second derivative of the logarithm of the likelihood function at its peak".

I haven't come across this definition before, and Google hasn't been particularly forthcoming; is this a standard definition, or something Kay has introduced himself?

This is in the context of a derivation of the CRLB.