Simplified Common Lisp reference
Symbol class: Mathematics, Arithmetics, Logic and Comparisons
Symbol type: function
notvalue => value
Argument description:
value logical value

NOT computes logical negation. Note that any other value than NIL is considered as true. NOT is identical to NULL, but used in conjunction with boolean logic processing unlike NOT which is used in list processing.

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