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)".