An index in Elasticsearch is a logical collection of documents, similar in concept to a database table. Each index groups related records so they can be searched, filtered, and analyzed efficiently. An Elasticsearch cluster may contain many indices, and some are system indices used by the platform itself while others store application data.
In cyber security, indices matter because they often contain logs, documents, credentials traces, or other sensitive operational data. If an attacker reaches the cluster, they may query a single index to steal a targeted data set or enumerate every non-system index to broaden the haul. Defenders should treat index-level access as a control boundary: restrict who can read or write each index, monitor unusual searches and exports, and make sure cleanup includes fixing the exposure that allowed access in the first place.