11.2 Variables that Never Change

In Emacs Lisp, certain symbols normally evaluate to themselves. These include nil and t, as well as any symbol whose name starts with `:' (these are called keywords). These symbols cannot be rebound, nor can their values be changed. Any attempt to set or bind nil or t signals a setting-constant error. The same is true for a keyword (a symbol whose name starts with `:'), if it is interned in the standard obarray, except that setting such a symbol to itself is not an error.

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

Function: keywordp object

function returns t if object is a symbol whose name starts with `:', interned in the standard obarray, and returns nil otherwise.