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.