AtomQ

AtomQ[expr] yields True if expr is an atomic expression that cannot be divided into subexpressions.

Examples

Numbers are atoms:

AtomQ[5]
(* True *)

Symbols are atoms:

AtomQ[x]
(* True *)

Lists are not atoms:

AtomQ[{1, 2, 3}]
(* False *)

Please visit the official Wolfram Language Reference for more details.