not x → boolean

x — a generalized boolean (i.e., any object).

boolean — a boolean.

Returns t if x is false; otherwise, returns nil.

(not nil) → T 
(not '()) → T 
(not (integerp 'sss)) → T 
(not (integerp 1)) → NIL 
(not 3.7) → NIL 
(not 'apple) → NIL

null

not is intended to be used to invert the ‘truth value’ of a boolean (or generalized boolean) whereas null is intended to be used to test for the empty list. Operationally, not and null compute the same result; which to use is a matter of style.