In set theory, a branch of mathematics, an urelement or ur-element (from the German prefix ur-, 'primordial') is an object (concrete or abstract) which is not a set, but that may be an element of a set. Urelements are sometimes called "atoms" or "individuals."

Read more about Urelement:  Theory, Urelements in Set Theory, Quine Atoms

Other articles related to "urelement, urelements":

Urelement - Quine Atoms
... An alternative approach to urelements is to consider them, instead of as a type of object other than sets, as a particular type of set ...