Here are three sources people can cite for the definition of locality by Peter Denning:

“The tendency for programs to cluster references to subsets of address space for extended periods is called the principle of locality” on page 143 in Peter J. Denning and Craig H. Martell. 2015. Great Principles of Computing. MIT Press

Locality is “the observed tendency of programs to cluster references to small subsets of their pages for extended intervals.” on page 23 in Peter J. Denning. 2005. The locality principle. Communications of ACM 48, 7 (2005), 19–24

Locality is “a pattern of computational behavior in which memory accesses are confined to locality sets for lengthy periods of time.” in an email from Denning,  June 26, 2016.

Given the domain name of this site, it is fitting to have the precise definition of the term here.

Locality has been discussed by others.

In their influential paper titled  Dynamic Storage Allocation: A Survey and Critical Review [ISMM 1995], Wilson and others wrote “Locality is very poorly understood, however; aside from making a few important general comments, we leave most issues of locality for future research.”