GNU Emacs Lisp Reference Manual

10.2: Variables That Never Change

Emacs Lisp has two special symbols, nil and t, that always evaluate to themselves. These symbols cannot be rebound, nor can their value cells be changed. An attempt to change the value of nil or t signals a setting-constant error.

nil == 'nil
     => nil
(setq nil 500)
error--> Attempt to set constant symbol: nil