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

Constraint Logic Programming

    (CLP) A programming framework based (like Prolog)
   on LUSH (or SLD) resolution, but in which unification has
   been replaced by a constraint solver.  A CLP interpreter
   contains a Prolog-like inference engine and an incremental
   constraint solver.  The engine sends constraints to the solver
   one at a time.  If the new constraint is consistent with the
   collected constraints it will be added to the set.  If it is
   inconsistent, it will cause the engine to backtrack.

   CLP* is a variant.

   ["Constraint Logic Programming", J. Jaffar et al, 14th POPL,
   ACM 1987].

   (1994-11-01)