Sphinx is a full-text search engine, distributed under GPL version 2. Commercial license is also available for embedded use.
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 sources support fetching data either via direct connection to MySQL or PostgreSQL, or using XML pipe mechanism (a pipe to indexer in special XML-based format which Sphinx recognizes).
As for the name, Sphinx is an acronym which is officially decoded as SQL Phrase Index. Yes, I know about CMU's Sphinx project.
G. Arumugam, M.Thangaraj, and R. Sasirekha. International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), 1, page 09-19. ACM, (April 2011)
M. Yahya, K. Berberich, S. Elbassuoni, M. Ramanath, V. Tresp, and G. Weikum. Proceedings of the 2012 Joint Conference on Empirical Methods in Natural Language Processing and Computational Natural Language Learning, page 379--390. Stroudsburg, PA, USA, Association for Computational Linguistics, (2012)
A. Jaffri. Proceedings of the 6th International Semantic Web Conference and 2nd Asian Semantic Web Conference (ISWC/ASWC2007), Busan, South Korea, volume 4825 of LNCS, page 915--919. Berlin, Heidelberg, Springer Verlag, (November 2007)