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

logic variable

    A variable in a logic programming language
   which is initially undefined ("unbound") but may get bound to
   a value or another logic variable during unification of the
   containing clause with the current goal.  The value to which
   it is bound may contain other variables which may themselves
   be bound or unbound.

   For example, when unifying the clause

   	sad(X) :- computer(X, ibmpc).

   with the goal

   	sad(billgates).

   the variable X will become bound to the atom "billgates"
   yielding the new subgoal "computer(billgates, ibmpc)".

   (1995-03-14)