The name given to the player who's controller is plugged into the first slot on a video game system. This title is usually only given out during a multiplayer game. Such a title is almost always argued/fought over, as it holds an unspoken degree of authority with it. Player 1 is usually chosen by allowing the best player or owner of the system to take this position.
"Hey, give me the 1st controller!"
"No! It's my system"
"But you suck at this game"
"Why does it matter?"
".... it just does"