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 in a definite clause, that means there is an implicit quantifier.
A typical first-order definite clause looks like this:
[[BROKEN LINK: cite:@ai_norvig_2020]]
in propositional logic, the clause is a definite clause, whereas is not, because it has two positive clauses.