Lockstep
Also said as Deterministic lockstep, Synchronous simulation
Every machine runs the entire game itself, and only orders cross the wire. Nobody moves on to the next turn until they have everyone's orders for it.
Thousands of units cost no more bandwidth than ten, which is how strategy games worked on dial-up modems. In return, the whole game must be identical on every machine, and the slowest player sets everyone's speed.
Say it
“Use deterministic lockstep with 100 ms turns, running orders two turns after they are given. Hide the delay with an immediate acknowledgement sound and a click marker, and send a checksum every turn.”
Where you’ve seen it
Age of Empires kept whole armies in step over 28.8k modems this way, as Paul Bettner and Mark Terrano described in 1500 Archers on a 28.8: a command issued now runs two turns later, so it can reach everyone first.
Not to be confused with rollback
Lockstep waits until it knows and never has to take anything back. Rollback carries on without knowing and takes it back when it was wrong.