In logic, a formula is said to be open if it contains at least one variable that does not also occur in some quantifier or abstraction in the formula, else it is said to be closed.

Open formulas are also sometimes identified with predicates or relations, especially in logic texts. Also, a common convention in logic is to leave out a string of universal quantifiers, as in '(Ez)(Rxyz)' for '(x)(y)(Ez)(Rxyz)'.