I would say clippy.el, and if Emacs is compiled with GTK+ support, it can display a "graphical" tooltip instead of a text one. Edit: I'm wrong, clippy describe

I would say clippy.el, and if Emacs is compiled with GTK+ support, it can display a "graphical" tooltip instead of a text one. Edit: I'm wrong, clippy describe a function in a tooltip, it doesn't show the definition, one should extend it to use something like xref-find-definition (M-.) for the Lisp mode (and for Elisp there is the elisp-slime-nav package).