Quorum
A minimum subset of replicas whose participation is sufficient to perform or validate an operation under a replication protocol.
Distributed-Systems Context
A quorum is a subset of replicas large enough to validate or perform an operation under a replication protocol. Majority quorums intersect, allowing information about earlier decisions to carry into later ones; read/write schemes may use relationships such as R + W > N for particular consistency goals.
Consistency Boundary
Quorum participation alone does not make a system linearizable. Read and write protocols, failure assumptions, version selection, and conflict resolution determine the actual consistency guarantee.