Saying the name of a famous person (celebrity, musician, actor, public figure, etc.) on a television show, or movie instead of that person physically appearing on the program, so they just have a line of dialog making it seem like a very famous person is getting ready to appear on the show, or is in the next room, or just off the cameras view.
On a crime drama a paparazzi sees a body in an alley near a high-end night club and goes around the corner to check it out. As he does, the camera goes off the pretty people and follows him. A fellow paparazzi hollers, "Hey dude, get back here; Kim & Khloe Kardashian just pulled up and got out of a Limo."
Doing a Nameo saves the show a large amount of money that they would otherwise have to pay out to get the famous people to do a cameo on the show.