Sphinx is a full-text search engine, distributed under GPL version 2.
Commercial licensing is also available upon request.

Generally, it's a standalone search engine, meant to provide fast,
size-efficient and relevant fulltext search functions to other
applications. Sphinx was specially designed to integrate well with SQL
databases and scripting languages. Currently built-in data source
drivers support fetching data either via direct connection to MySQL,
PostgreSQL, or from a pipe in a custom XML format.

As for the name, Sphinx is an acronym which is officially decoded as
SQL Phrase Index. Yes, I know about CMU's Sphinx project.