OpenSearch
Amazon's fork of Elasticsearch 7.10 — started January 2021 after Elasticsearch's licence change to SSPL/Elastic License. Apache 2.0 again.
Backed by AWS, later donated to the Linux Foundation in 2024. AWS OpenSearch Service is the managed offering. Builds increasingly divergent from Elasticsearch — its own kernels, ML features, security plugin.
Competitors: Elasticsearch (the original, still bigger ecosystem), Solr (Apache), Vespa (Yahoo), Meilisearch. OpenSearch wins at AWS shops and among open-source purists.