This is where a goal in the body of a rule is a compound term whose functor is a variable. By the time the goal is evaluated, the meta-variable must have been bound to an atom.
Example

map(Pred, [], []).
map(Pred, [X|Y], [X1|Y1]) :-
           Pred(X, X1),
           map(Pred, Y, Y1).

In this example, it is assumed that the meta-variable ’Pred’ will be bound to the name of a binary relation. Given the following binary relations:

double(X, Y) :-Y is X + X.

square(X, Y) :-Y is X * X.

map(double, [1,2,3,4], X).
X = [2,4,6,8]
map(square, [1,2,3,4], X).
X = [1,4,9,16]