Netcode
Also said as Network code, Online code, Multiplayer architecture
The code and the rules that keep players on different machines in the same game, when no two of them can ever see the same moment.
It decides how the whole game feels online. Added late, it usually means rewriting the game, because it reaches into how every single thing moves and who gets to decide what happened.
Say it
“Pick the netcode before we build combat. It is a two-player fighter on 60 frames, so I want peer to peer with rollback, a fixed tick and a game state small enough to save every frame.”
Where you’ve seen it
Riot's article Peeking Into Valorant's Netcode walks through a full set of these decisions: a server that decides what happened, clients that guess their own movement, and hits judged by rewinding to what the shooter saw.
Not to be confused with lag
Lag is what players feel. Netcode is what the game does about it. Good netcode can make a bad connection feel fine, and bad netcode can make a good one feel awful.