The dark web is part of the internet that isn't visible to search engines and requires the use of an anonymizing browser called Tor to be accessed.

Dark web definition

The dark web is a part of the internet that isn't indexed by search engines. You've no doubt heard talk of the “dark web” as a hotbed of criminal activity — and it is. Researchers Daniel Moore and Thomas Rid of King's College in London classified the contents of 2,723 live dark web sites over a five-week period in 2015 and found that 57% host illicit material. 

A 2019 study, Into the Web of Profit, conducted by Dr. Michael McGuires at the University of Surrey, shows that things have become worse. The number of dark web listings that could harm an enterprise has risen by 20% since 2016. Of all listings (excluding those selling drugs), 60% could potentially harm enterprises.