Merriam-Webster defines the term "photobomb" as ... well, absolutely nothing, because photobomb has yet to be added to the dictionary. It's more one of those "you know it when you see it" sort of things.

But in general, a photobomb is said to have taken place when someone (the photobomber) enters a photograph without the subjects of said picture (the photobombees) being aware of the person's presence. Silly faces are not required on the part of the bomber, but they are appreciated.

Bonus points if you do this to strangers and/or no one notices you until they view the photo later (or someone on Facebook eventually points out the creeper). Even bigger bonus points if it's a celebrity doing the bombing -- and you'll see plenty of that in this list.