defun is a Lisp macro in `byte-run.el'.

(defun NAME ARGLIST &optional DOCSTRING DECL &rest BODY)

Define NAME as a function.
The definition is (lambda ARGLIST [DOCSTRING] BODY...).
See also the function `interactive'.
DECL is a declaration, optional, of the form (declare DECLS...) where
DECLS is a list of elements of the form (PROP . VALUES). These are
interpreted according to `defun-declarations-alist'.
The return value is undefined.