Glossary search

deep web

(also invisible web), n. ~ Information, usually in databases, that is not directly accessible through a hyperlink and that cannot be indexed by most search engines.

Notes: 

Deep web content is often accessed through a form on a web page. A search engine may find the form, but cannot retrieve results because the form requires a search term.

Citations:
(Olsen 2004) [Yahoo!] introduced its Content Acquisition Program designed to index the billions of documents contained in public databases but that are commonly inaccessible to search engines, or what's called the invisible or deep Web.