empty sets

Noun

empty set (plural empty sets)

1. The unique set that contains no elements, denoted ∅ or {}.
   • Start with the empty set. We define it as “the set of all objects not equal to themselves,” since there are no such objects. All empty sets have the same members—no members at all! Therefore, as sets they're identical, by definition of identity of sets. In other words, there's only one empty set....1997, Reuben Hersh, What Is Mathematics, Really?, Oxford University Press, page 257

   Synonyms:

   null set