Lévy hierarchy

Etymology
Introduced by Azriel Lévy in 1965.

Proper noun

 * 1)  A hierarchy of formulas in the formal language of the Zermelo-Fraenkel set theory. Its first level contains only formulas with no unbounded quantifiers and is denoted by $$\Delta_0=\Sigma_0=\Pi_0$$. Subsequent levels are given by finding a formula in prenex normal form which is provably equivalent over ZFC, and counting the number of changes of quantifiers.