WordNet (r) 3.0 (2006):

drop-down menu
    n 1: a menu of options that appears below the item when the
         computer user clicks on it

The Free On-line Dictionary of Computing (30 December 2018):

pull-down menu
drop-down menu
pop-down menu

    (Or "drop-down menu", "pop-down menu") A
   menu in a graphical user interface, whose title is
   normally visible but whose contents are revealed only when the
   user activates it, normally by pressing the mouse button
   while the pointer is over the title, whereupon the menu
   items appear below the title.  The user may then select an
   item from the menu or click elsewhere, in either case the menu
   contents are hidden again.  A menu item is selected either by
   dragging the mouse from the menu title to the item and
   releasing or by clicking the title and then the item.

   When a pull-down menu appears in the main area of a window, as
   opposed to the menu bar, it may have a small,
   downward-pointing triangle to the right.

   Compare: scrollable list.

   (1999-09-22)