A definite clause is a logical statement characterised by a single positive proposition (called the head) and a 'body' consisting of a conjunction of positive or negative propsitions. It is a one-way implication that the head is true if the body is. For example:
is_grandfather(X,Y) ← is_father(X,Z) &land; is_child(Y,Z)
Typically there may be several clauses with the same proposition as their head, effectively defining a disjunction. These can also be used computationally in a definite clause grammar and for logic programming, notably in Prolog.