Elasticsearch is a distributed search and analytics engine made by Elastic. Distributed means the data and the work are spread over several servers (nodes) that act as one cluster, so the system grows by adding machines. Programs talk to it through a REST API, an interface of plain HTTP requests.
Elasticsearch stores structured, unstructured and vector data and makes new data searchable in near real time. It supports full-text search (matching words in documents, including fuzzy matching of misspelled terms), vector search over embeddings (lists of numbers that represent the meaning of text or images), and hybrid search that combines the two. The same engine runs log, metrics and security analytics, and Kibana adds dashboards on top of it. Client libraries exist for Java, Python, Go and other languages.
The source code is open under a choice of three licenses: GNU AGPL v3, Server Side Public License and Elastic License 2.0. It can be downloaded and self-managed, or run by Elastic as Elastic Cloud Hosted or Serverless, billed by resources or by usage.