In propositional calculus and in predicate calculus, an atomic sentence is an atomic formula which contains no variables. Otherwise the atomic formula is an open sentence.

As examples, let P, M, T be predicate letters; let a, b, c, etc. be constant terms; but let x, y, z be variable terms. Then these are atomic sentences:

• P
• M(a)
• T(b, c)
• P₂(b,a,c)

but these are not atomic sentences:

• M(x)
• T(a,z)
• P₂(x,y,z)

In prop.calc. all atomic formulas are atomic sentences, because atomic propositions correspond to 0-ary predicates, but a predicate must be at least unary in order to be able to introduce variable terms.