Delta compression
Sends only the fields that changed since the client's last acknowledged snapshot instead of the full world state every tick.
QuakeWorld (1996) and later the Source engine made the technique standard in action games. The server keeps a history of the snapshots each client has acknowledged and sends only the difference — the delta — against that baseline. Since most of a world sits still between two ticks, bandwidth drops dramatically.
If the acknowledgement is lost — packets do go missing — the server falls back to a full, non-delta update until the client is back in sync. The delta is usually paired with snapshot interpolation so motion looks smooth despite sparse updates.