Split screen
Also said as Couch co-op, Local multiplayer, Shared screen
Players sit in one room, on one machine. Either they each get a slice of the television, or they share one view that has to keep all of them in it.
No netcode, no lag, and you can see your friend's face. But the game has to draw itself several times over, and each player gets less of the screen, so it is an expensive thing to add late.
Say it
“Four-player split screen, running at 30 FPS. Cap the draw distance at 60% and turn off shadows on the other players' views so that we stay inside the frame budget.”
Where you’ve seen it
Mario Kart 64 divided the television between four players, and some reviewers complained that its quadrants were too small to see. It Takes Two uses a split view even when you play it online.
Not to be confused with online co-op
Online co-op is people on separate machines, with all the problems in this world. Couch co-op is one machine, and has none of them.