Stream sniping is when someone watches your stream to get an upper hand to essentially cheat against you in a game. For example, they could find your position on an open-world map, find out where your base is, or even find out when you're low on health, so they can ambush you. It sucks. The deveopers of Rust, Facepunch Studios... are (to my knowledge), the first game developers to build a preventative measure against it into their game. They call it Streamer Mode.
Hey, its Edward Longface again! Ahhhh!!! Why did he shoot at me?! He probably knew my plan. He must be stream sniping!
to follow a person who is streaminggameplay footage (usually on twitch or ustream) into the same games, using their stream to find and harass them (often for hilarious results.)
"Do a can crush for Jay Owens in 30 seconds or you will be shot.... Oh my god, it's the same guy! He's stream sniping me!"
One who watches a Fortnite stream, usually on twitch or mixer, ques up in the same game, kills the streamer and taunts them in some way to get attention.
When a person, usually famous is streaming a multiplayervideo game and a person in the audience tries to get into the same game that the streamer is in and targets them, or tries to somehow talk to them.