head
The first part of a Prolog rule. It is separated from the body by the neck symbol :-. It normally has the form of a functor (i.e. a relation symbol, followed by a comma-separated list of parameters, in parentheses. E.g. in the rule

sister_of(X,Y) :-
    female(Y),
    X \== Y,
    same_parents(X,Y).
sister_of(X,Y) is the head.