A black budget is a government budget that is allocated for classified or other secret operations of a nation. The black budget is an account expenses and spending related to military research and covert operations. The black budget is mostly classified due to security reasons. The black budget can be complicated to calculate, but in the United States it has been estimated to be over US$50 billion a year.
The United States Department of Defense has a black budget it uses to fund black projects--expenditures that it does not want to disclose publicly. The annual cost of the United States Department of Defense black budget was estimated at $30 billion in 2008, but was increased to an estimated $50 billion in 2009. A black budget article by The Washington Post, based on information given by Edward Snowden, detailed how the US allocated $52.8 billion in 2012 for the black budget.
The black budget has been known to hide multiple types of projects from elected officials. With secret code names and hidden figures, the details of the black budget are revealed only to certain people of congress, if at all.
This budget was approved by the US National Security Act of 1947, which created the Central Intelligence Agency, the National Security Council and reorganized some military bases with help of the Defense Department.
The U.S. Government claims that the money given to this budget investigates advanced sciences and technologies for military issues. This kind of research is responsible for the creation of new aircraft, weapons, and satellites.
In 2018, some newspapers reported that the Trump administration asked for $81.1 billion for the 2019 black budget. The request included $59.9 billion for the National Intelligence Program, covering non-military programs and activities. Also $21.2 billion for the Military Intelligence Program which covers Intelligence activities by the Department of Defense.  "In total, these two are more than 3.4% higher than the FY2018 request and the largest since then... [and it's] the largest announced since the government began disclosing its intelligence budget request in 2007..." according to The Washington Times Andrew Blake.
According to an estimate by the Moscow-based Gaidar Institute, about 21% (3.2 trillion rubles) of the Russian federal budget was "black" (not itemized) in 2015. This represents a doubling from 2010. The increase coincided with huge increases in the Russian military budget under Russian President Vladimir Putin.