The simpler (and original) way to define a menu item is to bind some event type (it doesn't matter what event type) to a binding like this:
(item-string . real-binding)
The car, item-string, is the string to be displayed in the menu. It should be short—preferably one to three words. It should describe the action of the command it corresponds to. Note that not all graphical toolkits can display non-ASCII text in menus (it will work for keyboard menus and will work to a large extent with the GTK+ toolkit).
You can also supply a second string, called the help string, as follows:
(item-string help . real-binding)
help specifies a help-echo string to display while the mouse
is on that item in the same way as
help-echo text properties
(see Help display).
As far as
define-key is concerned, item-string and
help-string are part of the event's binding. However,
lookup-key returns just real-binding, and only
real-binding is used for executing the key.
If real-binding is
nil, then item-string appears in
the menu but cannot be selected.
If real-binding is a symbol and has a non-
menu-enable property, that property is an expression that
controls whether the menu item is enabled. Every time the keymap is
used to display a menu, Emacs evaluates the expression, and it enables
the menu item only if the expression's value is non-
nil. When a
menu item is disabled, it is displayed in a fuzzy fashion, and
cannot be selected.
The menu bar does not recalculate which items are enabled every time you
look at a menu. This is because the X toolkit requires the whole tree
of menus in advance. To force recalculation of the menu bar, call
force-mode-line-update (see Mode Line Format).