Co-op
Also said as Cooperative play, Co-operative, Team play
Players on the same side, against the game. Good co-op makes them need each other, not just share a screen or a map.
Without some reason to rely on one another, co-op is several people playing a single-player game in the same room. A rule that makes you stop and help is what turns them into a team.
Say it
“Downed players bleed out over 30 s and only a teammate can revive them, in 3 s. A revive leaves the helper open, so it must be a real decision in the middle of a fight.”
Where you’ve seen it
In Left 4 Dead, a survivor out of health can only be helped by another survivor, and a Hunter holds its victim down until someone pushes it off. It Takes Two has no single-player option at all.
Not to be confused with multiplayer
Multiplayer is any game with more than one player. Co-op is a kind of it, in which they are on one side.