or -definable, or definable with parameters from cf. below) if there is a formula such that

In other words, is definable if and only if there is a formula such that

is correct.

An important special case is the definability of specific elements. An element of is definable in if and only if there is a formula such that

Definability with parameters

A relation is said to be definable with parameters (or -definable) if there is a formula with parameters from such that