The minimum number of nodes that must agree for a distributed system to make progress, the basis of consensus protocols.
A quorum is the smallest subset of nodes in a distributed system whose agreement is required to make a decision, write a value, elect a leader, commit a transaction. For an odd-sized cluster with N nodes, the quorum is typically (N/2)+1: 2 of 3, 3 of 5, 4 of 7. Quorum sizing is what gives consensus protocols (Raft, Paxos) their fault tolerance: a 5-node cluster with quorum-3 can lose 2 nodes and still make progress.
Cluster sizing decisions are quorum-sizing decisions. A 2-node cluster has no useful quorum (any 1 node failure loses majority). A 3-node cluster tolerates 1 failure. A 5-node cluster tolerates 2. Adding a 6th node hurts: even-numbered clusters have worse fault tolerance per node than odd-numbered. Teams that size by gut feel often pick even numbers and end up surprised at the blast radius of a single failure.
See the part of the platform that handles quorum in production.