Simplified Common Lisp reference
Symbol class: Mathematics, Arithmetics, Logic and Comparisons
Symbol type: symbol
nil => symbol

NIL symbol denotes empty list and false value. There is also ubiquitous NIL constant which contains NIL symbol. NIL is considered as false value by comparison functions and control operators (unlike any other). Empty list, that is '() or even (), is the same as NIL value.

nil => NIL
'nil => NIL
() => NIL
'() => NIL
(eq nil 'nil) => T
(eq 'nil ()) => T
(eq () '()) => T
(not t) => NIL
(not nil) => T
(not 234.3) => NIL
Function indexFull documentation for nil (HyperSpec)