The deep web (or hidden web, or invisible web) is that part of the World Wide Web that is not accessible to search engines. This includes proprietary corporate data, confidential public data protected by government regulation, commercial information accessible only to selected groups of people (e.g. subscriber-only sites), private e-mail, private social media content, etc. The part of the web that is indexed is known as the surface web. It has been estimated that the surface web makes up only 1 per cent of all web content.

The term deep web should not be confused with dark net or dark web.