Quorum
The minimum number of replicas that must respond for an operation to be considered committed, sized so that any read set and any write set must overlap.
The arithmetic is what makes quorums useful and is easy to state: with N replicas, if the write quorum W and read quorum R satisfy W + R > N, then every read set intersects every write set, so a read is guaranteed to see at least one replica carrying the latest write.
That single inequality is a tunable trade rather than a fixed rule. W = N and R = 1 gives fast reads and slow, fragile writes. W = 1 and R = N is the reverse. W = R = ⌈(N+1)/2⌉ — a majority — balances both and is what consensus protocols use, because a majority quorum also guarantees that any two quorums overlap, which is what prevents split-brain.
The consequences worth carrying into design reviews. An odd number of replicas is strictly better than the next even number: five nodes tolerate two failures, six also tolerate two, so the sixth adds cost and no fault tolerance. Two nodes cannot form a majority quorum at all, which is why a two-region active-active deployment needs a third site as a witness — the most common gap in two-region designs.
And the latency of a quorum write is the latency of the slowest member of the fastest majority, so replica placement across regions directly sets your write latency floor, which is physics rather than configuration.