Simplified Common Lisp reference
atom
Symbol class: Symbol, Characters, Hash, Structure, Objects and Conversions
Syntax:
Symbol type: function
atomobject => T or NIL
Argument description:
object an object

ATOM function returns true if the argument is not a cons cell, otherwise it returns false. See CONS and LIST.

(atom nil) => T
(atom 'some-symbol) => T
(atom 3) => T
(atom "moo") => T
(atom (cons 1 2)) => NIL
(atom '(1 . 2)) => NIL
(atom '(1 2 3 4)) => NIL
(atom (list 1 2 3 4)) => NIL
Function indexFull documentation for atom (HyperSpec)