A program that indexes documents, then attempts to match documents relevant to a user’s search requests.
The term search engine is most commonly used to refer to Web search engines, although other types of search engines exist. Web search engines attempt to index a large portion of pages on the World Wide Web. Other search engines are topic-specific, region-specific, and even site-specfic.
There is also some confusion about the different between a search engine and a search destination. A search engine powers the search process and provides results for a search destination. A search destination can use its own engine, a 3rd-party engine, or a combination. Knowing the difference between an engine and a destination is important when submitting URLs; a destination using its own engine can accept direct submissions, while a destination using external engines may or may not provide a submission option.
see also: SEO Tools