Common Lisp the Language, 2nd Edition
The names nil and t are constants in Common Lisp. Although they are symbols like any other symbols, and appear to be treated as variables when evaluated, it is not permitted to modify their values. See defconstant.
[Constant]
nil
The value of nil is always nil. This object represents the logical false value and also the empty list. It can also be written ().
[Constant]
t
The value of t is always t.