The term dark web is a general and rather imprecise term used to refer to sites and services that are not directly accessible on the internet. Access to such sites is typically only provided through gateway services or proxies. One common example would be sites that are only accessible using the TOR anonymizing service. These services are variously used to either control or restrict access, or to ensure the anonymity of both site ownership and location, as well as visitor identity.
Strictly speaking, if a site is directly accessible via an http or https connection it’s not considered to be on the dark web. Sites which have simply removed themselves from search engine results, but remain on the public internet, are classified by the equally general and imprecise term deep web.