Of the materials that can be found on the WWW, only a small part can be freely accessed and found with search engines. The inaccessible part of the web is called the invisible web (or deep web, hidden web). The invisible web is protected by passwords and/or firewalls and it is not meant for public use. The contents of many catalogues and databases also cannot be found with search engines, although these materials are meant for public use. From these databases, materials can be retrieved only by using the search engine of the individual database. Therefore, they, too, can be considered as a part of the invisible web.
The invisible web includes
- materials protected with a password
- documents and interactive tools (e.g. dictionaries, reference works) protected with a firewall
- materials held in intranets
- databases that are accessible through a dialogue window (e.g. some archives, document databases, libaray catalogues).
In order to find materials meant for public use, one has to use subject portals as well as search engines.