Quorum
A minimum number of nodes that must acknowledge an operation for it to count, chosen so that read and write sets are guaranteed to overlap.
With N replicas, a write acknowledged by W nodes and a read served from R nodes will always see the latest write if W + R > N. That inequality is the whole mechanism.
It gives a tuning dial rather than a binary choice. N=3, W=3, R=1 makes reads fast and writes fragile (any node down blocks writes). N=3, W=2, R=2 tolerates one node failing for both. N=3, W=1, R=1 is fast, available, and eventually consistent — the overlap guarantee no longer holds.
Two caveats that matter in practice: quorum requires a majority to make progress, so a two-node deployment cannot form one and needs a third witness. And "latest" requires a way to order concurrent writes — timestamps, version vectors or a consensus protocol — or you have overlap without a way to say which value wins.