I have never saw something like a "definition" of NL.
I found very useful the concepts about NL:
Luisa Mich, Mariangela French, Pierluigi Novi Inverardi, Market research for requirements analysis using linguistic tools, Requirements Engineering
February 2004, Volume 9, Issue 1, pp 40-56.
There is a book from other field than Software Engineering with an interesting analysis of NL: Umberto Eco, La búsqueda de la lengua perfecta, chap 1. (I am sorry I don't know about an English version)