Tests are done at different defined levels with varying purposes. Unit, integration and system are common test levels.

ISTQB: A specific instantiation of a test process.

Tests are done at different defined levels with varying purposes, often starting from a somewhat atomistic approach (low-level) and evolving to a holistic approach (high-level). Test levels capture this idea.

Test levels can also apply to different levels between simple and complex, where a smoke test is an example of a very low level simple set of tests.

Test levels is a valid term when talking about different polarities and levels inbetween.