definite clause

definite clauses are disjunctions of literals of which exactly one is positive. existential quantifiers are not allowed, and universal quantifiers are left implicit: if you see an $\text{[math]}$ in a definite clause, that means there is an implicit $\text{[math]}$ quantifier.

A typical first-order definite clause looks like this:
$\text{[math]}$

[broken link: cite:@ai_norvig_2020]

in propositional logic, the clause $\text{[math]}$ is a definite clause, whereas $\text{[math]}$ is not, because it has two positive clauses.