Atomic formula of a set?

A series of questions asks me about the "atomic formula" (AF) of a set {A, B, C}. "Atomic formula" isn't defined anywhere in the text nor can I find information on what it is online. Is "atomic formula" a standard math term? The only clue the book gives is that AF$_0$ = {A, B, C}. What is AF$_1$ then? Or AF$_2$?

Thanks!

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 The notion is very language dependent. For the propositional calculus, they are the proposition letters, augmented sometimes by the symbols $0$ and/or $1$. For the predicate calculus, one needs to first define term. Then the atomic formulas are $t_1=t_2$ and $P(t_1,\dots,t_n)$ where $P$ ranges over the $n$-ary predicate symbols, and the $t_i$ range over the terms. – André Nicolas Oct 8 '12 at 0:22