WordNet (r) 3.0 (2006):

set theory
    n 1: the branch of pure mathematics that deals with the nature
         and relations of sets

The Free On-line Dictionary of Computing (30 December 2018):

set theory

    A mathematical formalisation of the theory of
   "sets" (aggregates or collections) of objects ("elements" or
   "members").  Many mathematicians use set theory as the basis
   for all other mathematics.

   Mathematicians began to realise toward the end of the 19th
   century that just doing "the obvious thing" with sets led to
   embarrassing paradoxes, the most famous being Russell's
   Paradox.  As a result, they acknowledged the need for a
   suitable axiomatisation for talking about sets.  Numerous
   such axiomatisations exist; the most popular among ordinary
   mathematicians is Zermelo Fränkel set theory.

   The beginnings of set theory
   (http://www-groups.dcs.st-and.ac.uk/~history/HistoryTopics.html).

   (1995-05-10)