The portion of the Web not accessible through Web search engines.
The invisible Web sounds very mysterious, but surfers visit it regularly, perhaps without even realizing it. “Invisible” Web pages are often dynamic pages within database-driven sites, whereas much of the “surface Web” consists of static pages. The URLs can often be identified by symbols such as the question mark (?).
Search engines are well aware of the invisible Web, but limitations in time and money could make it unfeasible to index all of the dynamic page possibilities. Even if resources were no issue, dynamic pages tend to present more challenges than static pages in terms of ranking useful results.
Most of the dynamic, database-driven sites in question do feature site search, so unearthing information becomes much easier if you can pinpoint the most appropriate site to search.
Invisible Web Gets Deeper
Search Engine Watch (August 2, 2000)
Invisible Web Revealed
Search Engine Watch (July 6, 1999)