urelement

Etymology
From.

Noun

 * 1)  A mathematical object which is not a set but which can be an element of a set.
 * 2) * 1996, Scientific Books staff (translators),, Definability and Computability, Scientific Books, page viii,
 * The introduction of urelements would seem to be a technical improvement; however, now we know that just such an extension of the notion of the admissible set led to the universal theory of computability based on the notion of definability by formulas with (in a broad sense) effective semantics.
 * The introduction of urelements would seem to be a technical improvement; however, now we know that just such an extension of the notion of the admissible set led to the universal theory of computability based on the notion of definability by formulas with (in a broad sense) effective semantics.

Usage notes
The standard axiomatisation of set theory, ZFC, ignores urelements. (By the axiom of extensionality, two sets whose only difference is that one contains urelements which the other does not would be equal.)

Translations

 * Icelandic: frumstak
 * Italian: urelemento