Delta snapshot
In multiplayer netcode, sending only the difference from a previously acknowledged snapshot instead of the full world state every tick.
Shipping the whole game world to every client many times a second quickly becomes untenable. Delta compression, which Quake 3 and later the Source engine made standard practice, instead sends only the fields that changed since the snapshot the client last acknowledged. If nothing moves, almost nothing is sent.
The cost is bookkeeping: the server must remember which baseline each client last acked in order to compute the delta against that one. Lose a packet and the client falls back on an older baseline until a fresh ack arrives. The method is the ground that snapshot interpolation and entity interpolation rest on.