Atomic formula

Atomic formula is a formula without connectives or quantifiers, made up only of a predicate with arguments.

Examples

P(a) or R(x,y)

More complicated formulas like [P(a) & R(x,y)] or All(x)[P(x)] are not atomic formulas due to the presence of & and All.

Link

Utrecht Lexicon of Linguistics

Reference

Gamut, L.T.F. 1991. Logic, language, and meaning. Chicago: Univ. of Chicago Press.