A clause in Prolog is a unit of information in a Prolog program ending with a full stop ("."). A clause may be a fact, like:

likes(mary, pizza).

or a rule, like:

eats(Person, Thing) :- likes(Person, Thing), food(Thing).

A clause may also be a query to the Prolog interpreter, as in:

?- eats(mary, pizza).

A group of clauses about the same relation is termed a procedure.