1. In propositional languages, a set $\left\{p_0,...,p_n,...\right\}$ of sentence letters, each of which serves as a stand-in for a simple proposition.

2. In first-order languages, a set of simple formulae of the form $R{t}_0,...,t_{n-1}$ for an $n$-ary relation symbol $R$ and terms $t_0,...,t_{n-1}$ (if identity is taken to be a logical symbol, then equational formulae of the form $t_0=t_1$ are considered atoms as well). The precise extension of a set of atoms is peculiar to a signature $\sigma$ describing the relation symbols and terms from which a set of atomic formulae is constructed.