What Does Photobomb Mean?
A photobomb occurs when a third party interferes with a photo in a way that shifts the focus from the original subject(s) to the photobomber. Photobombing can be as simple as jumping into the frame as the picture is taken, or may involve an elaborate setup with costumes and props. These actions are referred to as a photobomb because they usually ruin the photo in the eyes of the intended subjects.
Techopedia Explains Photobomb
The ability to share successful hijackings through social media has made photobombing an Internet meme. The range of photobombing techniques has expanded as social sharing has introduced variations. The most common photobomb is usually a form of photo lurking where a photobomber mugs for the camera in a shot he or she isn’t supposed to be part of (wedding photos, tourist photos and so on). More elaborate background setups include looking threatening and/or brandishing a weapon, mooning or flashing the camera, appearing in costume and so on.