A black operation or black op is a covert or clandestine operation by a government agency, a military unit or a paramilitary organization; it can include activities by private companies or groups. Key features of a black operation are that it is secret and it is not attributable to the organization carrying it out.
A single such activity may be called a black bag operation; that term is primarily used for covert or clandestine surreptitious entries into structures to obtain information for human intelligence operations. Such operations are known to have been carried out by the FBI, CIA, KGB, Mossad, MI6, MI5, ASIS, COMANF, DGSE, AISE, CNI, MSS, R&AW, DGFI, SVR, FSB, Kuwait 25th Commando Brigade and the intelligence services of other nations.
The main difference between a black operation and one that is merely secret is that a black operation involves a significant degree of deception, to conceal who is behind it or to make it appear that some other entity is responsible (e.g. false flag operations).
Black may be used as a generic term for any government activity that is hidden or secret. For example, in the United States, some activities by military and intelligence agencies are funded by a classified "black budget", of which the details, and sometimes even the total, are hidden from the public and from most congressional oversight.