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

namespace

    The set of all possible identifiers for some kind
   of object.  From the definition of a set, all names in a
   namespace are unique and there is some rule to determine
   whether a potential name is an element of the set.  For
   example, the Domain Name System includes rules for
   determining what constitutes a valid host name.

   (2008-12-09)