A sentential formula that contains at least one free variable (Carnap 1958, p. 24). A sentential variable containing no free variables (i.e., all variables are bound) is called a closed sentential formula. Examples of open sentential formulas include
which means that is even (over the domain of integers), and
which means that and is not the product of two numbers (both greater than one), i.e., is prime.
Closed sentential formulas are known as sentences, although it sometimes also happens that open sentential formulas are admitted as sentences (Carnap 1958, p. 25).