The same name
Giving a character a certain name is one of the most obvious ways of referencing something. For instance, Harry Potter fans knew that Voldemort was the evil bad guy, simply because his name sounds like the name of a villain. Also, Dr. Doom? Could it be more obvious? And sometimes, the names of characters can come from people behind the scenes.
For instance, we’re introduced to President Ellis in Iron Man 3, the 45th POTUS in the Marvel Universe. This name may not ring any bells, but hardcore Marvel fans will know that this is a shoutout to Warren Ellis, one of the freelance authors for the Iron Man comics.