An open secret is a concept or idea that is "officially" secret or restricted in knowledge, but is actually widely known; or refers to something which is widely known to be true, but which none of the people most intimately concerned are willing to categorically acknowledge in public.

Examples of military open secrets:

• Area 51
  Area 51
  Area 51 is a military base, and a remote detachment of Edwards Air Force Base. It is located in the southern portion of Nevada in the western United States, 83 miles north-northwest of downtown Las Vegas. Situated at its center, on the southern shore of Groom Lake, is a large military airfield...
  
  
• Camp Mirage
  Camp Mirage
  Camp Mirage is the codename for a former Canadian Forces forward logistics facility located in Dubai, United Arab Emirates. The facility was established in late-December 2001 and, though not officially acknowledged by the Canadian Forces, is considered an "open secret"...
  
  
• DARPA
• Delta Force
  Delta Force
  1st Special Forces Operational Detachment-Delta is one of the United States' secretive Tier One counter-terrorism and Special Mission Units. Commonly known as Delta Force, Delta, or The Unit, it was formed under the designation 1st SFOD-D, and is officially referred to by the Department of Defense...
  
  
• DEVGRU
• ECHELON
  ECHELON
  ECHELON is a name used in global media and in popular culture to describe a signals intelligence collection and analysis network operated on behalf of the five signatory states to the UK–USA Security Agreement...
  
  
• Electromagnetic bomb
• Nuclear weapons and Israel
  Nuclear weapons and Israel
  Israel is widely believed to be the sixth country in the world to have developed nuclear weapons and to be one of four nuclear-armed countries not recognized as a Nuclear Weapons State by the Nuclear Non-Proliferation Treaty , the others being India, Pakistan and North Korea...
  
  
• Numbers station
  Numbers station
  A numbers station is a shortwave radio station of uncertain origin. In the 1950s, Time magazine reported that the numbers stations first appeared shortly after World War II and were using a format that had been used to send weather data during that war.Numbers stations generally broadcast...
  
  s
• SOSUS
  SOSUS
  SOSUS, an acronym for Sound Surveillance System, is a chain of underwater listening posts across the northern Atlantic Ocean near Greenland, Iceland and the United Kingdom — the GIUK gap. It was originally operated by the United States Navy for tracking Soviet submarines, which had to pass...
  
  

The BT Tower

BT Tower

The BT Tower is a tall cylindrical building in London, United Kingdom, located at 60 Cleveland Street, Fitzrovia W1T 4JZ, London Borough of Camden. It has been previously known as the Post Office Tower, the London Telecom Tower and the British Telecom Tower. The main structure is tall, with a...

, despite being a 177 metres (580.7 ft) tall structure in the middle of central London, was an official secret and did not appear on Ordnance Survey maps until it was officially revealed by Kate Hoey

Kate Hoey

Catharine Letitia Hoey is a British Labour Party politician who has been the Member of Parliament for Vauxhall since 1989. She served in the Blair Government as Minister for Sport from 1999 to 2001.-Background:...

 under parliamentary privilege

Parliamentary privilege

Parliamentary privilege is a legal immunity enjoyed by members of certain legislatures, in which legislators are granted protection against civil or criminal liability for actions done or statements made related to one's duties as a legislator. It is common in countries whose constitutions are...

 in 1993.

Kayfabe

Kayfabe

In professional wrestling, kayfabe is the portrayal of events within the industry as "real" or "true". Specifically, the portrayal of professional wrestling, in particular the competition and rivalries between participants, as being genuine or not of a worked nature...

, or the presentation of professional wrestling

Professional wrestling

Professional wrestling is a mode of spectacle, combining athletics and theatrical performance.Roland Barthes, "The World of Wrestling", Mythologies, 1957 It takes the form of events, held by touring companies, which mimic a title match combat sport...

 as "real" or unscripted, is an open secret, kept displayed as legitimate within the confines of wrestling programs but openly acknowledged as predetermined by wrestlers and promoters in the context of interviews for decades.

An example of an open secret in politics

Politics

Politics is a process by which groups of people make collective decisions. The term is generally applied to the art or science of running governmental or state affairs, including behavior within civil governments, but also applies to institutions, fields, and special interest groups such as the...

 is that it may be widely known that an individual government minister holds a particular opinion, but is at present unable to express that opinion publicly because it is contrary to the formally expressed view of the government of which he or she is a member.

In television, the primary real-world identity of The Stig

The Stig

The Stig is a character in the British motoring television show Top Gear. The character plays on the anonymity of racing drivers' full-face helmets, with the running joke that nobody knows who, or indeed what, is inside the character's racing suit. The character was the creation of presenter Jeremy...

, a costumed and masked television test-driver used by BBC Television

BBC Television

BBC Television is a service of the British Broadcasting Corporation. The corporation, which has operated in the United Kingdom under the terms of a Royal Charter since 1927, has produced television programmes from its own studios since 1932, although the start of its regular service of television...

 for Top Gear, was an open secret until the unofficial embargo was broken by a newspaper in 2009.