Horn Clause

A clause (i.e., a disjunction of literals) is called a Horn clause if it contains at most one positive literal. Horn clauses are usually written as

or

where and is the only positive literal.

A definite clause is a Horn clause that has exactly one positive literal. A Horn clause without a positive literal is called a goal.

Horn clauses express a subset of statements of first-order logic. Programming language Prolog is built on top of Horn clauses. Prolog programs are comprised of definite clauses and any question in Prolog is a goal.