What is "urelement"

n. (context set theory English) A mathematical object which is not a set but which can be an element of a set.

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