Black project is an informal term used to describe a highly classified, top-secret military or defense project that is not publicly acknowledged by government, military personnel, or contractors.
In the United States, the formal term for a black project is an unacknowledged special access program (SAP). Black projects receive their funding from the black budget.
The US depends on private defense contractors to develop and build military equipment. The two most notable examples are Lockheed Martin and Northrop Grumman. The R&D department of Lockheed Martin is commonly referred to as Skunk Works; it is responsible for a number of aircraft designs, highly classified R&D programs, and exotic aircraft platforms.
Two well known sites for testing of black projects are the Nevada Test site and Area 51.
Below are examples of previously unacknowledged black projects categorized per country.