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

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

Define NAME as a macro.
When the macro is called, as in (NAME ARGS...),
the function (lambda ARGLIST BODY...) is applied to
the list ARGS... as it appears in the expression,
and the result should be a form to be evaluated instead of the original.
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 `macro-declarations-alist'.
The return value is undefined.