Matchmaking
Also said as Matchmaker, Queue, SBMM, Skill-based matchmaking
The system that picks who plays whom, trading how long you wait against how fair the game will be, and how good the connection is.
A strict search gives close games and long queues, and a loose one gives the opposite. Fewer players makes both worse, so a shrinking game loses people faster and faster, unless the rules are changed.
Say it
“Start by accepting a skill gap of 50 points and widen it by 10% each second, capped at 400. Never put anyone over 120 ms of ping, and show the expected wait.”
Where you’ve seen it
Halo 2 replaced manual lobbies with playlists: you picked a type of match, and the game chose the map and mode, found other players and ranked your skill. Bungie's own testers at first preferred traditional servers.
Not to be confused with server browser
With a server browser, players choose a game themselves. With matchmaking, the system chooses for them, and most players never see how.