The Collaborative International Dictionary of English v.0.48:

Universe of discourse \U"ni*verse of dis"course\, n.
   everything which may be considered, explicitly or implicitly,
   in a particular theory or discussion.
   [PJC]

WordNet (r) 3.0 (2006):

universe of discourse
    n 1: everything stated or assumed in a given discussion [syn:
         universe, universe of discourse]

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

universe of discourse

    In ontology, the set of all entities
   that can be represented in some declarative language or other
   formal system.

   Each entity is represented by a name and may have some
   human-readable description of its meaning.  Formal axioms
   constrain the interpretation and well-formed use of these
   names.

   (2005-07-29)