TL;DR

This post gives definition of nonfunctional testing that is aligned with Black Box Software Testing Foundations course (BBST) created by Rebecca Fiedler, Cem Kaner and James Bach.

Definition

In contrast to functional testing, software developers coined clumsy term: nonfunctional testing. So we have statements:

All nonfunctional tests are working.

Nonfunctional testing is testing that involves all software attributes that are not specific to any software function, but are general.

Example

Security testing is example of nonfunctional testing. We expect that all software functions are secure.