A group of blockchains that can communicate with each other in a trust-minimized way. Trust-minimized communication can be facilitated by fraud or validity proofs. Chains that can’t communicate in a trust-minimized way are in separate clusters that rely on trust-based communication.
For example, Ethereum and its rollups are a cluster of chains, and standalone layer 1s, such as Polygon, each sit in their own cluster.
Read more about the concept of clusters.