Source : Free On-Line Dictionary of Computing

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)