Elasticsearch is an open-source, distributed search and analytics engine built on Apache Lucene. Originally released in 2010, it excels at full-text search, log analytics, and real-time data analysis across petabyte-scale datasets, powering use cases from application search to security analytics. At its core, Elasticsearch uses an inverted index that maps terms to documents, enabling sub-second query responses even when searching billions of records distributed across cluster nodes. Understanding how queries interact with mappings, analyzers, and aggregations is essential — a poorly designed mapping or analyzer can turn a millisecond query into a multi-second timeout.