(mark-defun &optional ARG)
Put mark at end of this defun, point at beginning.
The defun marked is the one that contains point or follows point.
With positive ARG, mark this and that many next defuns; with negative
ARG, change the direction of marking.
If the mark is active, it marks the next or previous defun(s) after
the one(s) already marked.
Emacs 26 changes the signature of
mark-defun
: