Definable number

A real number a is first-order definable in the language of set theory, without parameters, if there is a formula φ in the language of set theory, with one free variable, such that a is the unique real number such that φ(a) holds (in the von Neumann universe V).

Related Topics:
Real number - Set theory - Free variable - Von Neumann universe

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For the purposes of this article, such reals will be called simply definable numbers. This should not be understood to be standard terminology.

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