A cluster hosts one or more indexes and is responsible for providing operations such as searching, indexing, and aggregations. A cluster is formed by one or more nodes. Every Elasticsearch node is always part of a cluster, even if it is just a single node cluster. By default, every Elasticsearch node tries to join a cluster with the name Elasticsearch. If you start multiple nodes on the same network without modifying the cluster.name property in config/elasticsearch.yml, they form a cluster automatically.
It is advisable to modify the cluster.name property in the Elasticsearch configuration file to avoid joining another cluster in the same network. Since the default behavior of a node is to join an existing cluster within the network, your local node may try to join another node and form a cluster. This can happen in developer machines and also in other environments as long as the nodes are in the same network.
A cluster consists of multiple nodes, where each node takes responsibility for storing and managing its share of data. One cluster can host one or more indexes. An index is a logical grouping of related types of documents.