Open secret

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.

Government and military
One famous "open secret" is that of Area 51, a United States military base containing an aircraft testing facility. The U.S. Government did not explicitly affirm the existence of any military facility near Groom Lake, Lincoln County, Nevada until 2013, when the CIA released documents revealing that the site was established to test spy planes. While the general location of the base is now officially acknowledged, the base does not appear on government maps or in declassified satellite photography. Yet despite this, the base was demonstrably and widely acknowledged to exist for many years before the CIA officially confirmed its existence. The immense secrecy has made it the frequent subject of conspiracy theories and a central component to UFO folklore.

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, was considered an open secret.

The existence of the British Secret Intelligence Service (MI6) was widely known for several decades before the government's official acknowledgement of the organisation in 1994. Likewise, Delta Force can be considered an open secret, since its existence has been denied in the past by the government.

Israel is widely acknowledged to possess nuclear weapons. This can be considered an open secret, because the Israeli government has never explicitly stated that it possesses a nuclear stockpile, officially maintaining a policy of deliberate ambiguity.

Completed in 1964, the Post Office Tower was an official secret and did not appear on Ordnance Survey maps until after it was officially revealed by Kate Hoey under parliamentary privilege in 1993, despite being a 177 m tall structure in the middle of central London that was open to the public for about 15 years.

Entertainment
Kayfabe, or the presentation of professional wrestling 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.

In television, the primary real-world identity of The Stig, a costumed and masked television test-driver used by BBC Television for Top Gear, was an open secret until the unofficial embargo was broken by a newspaper in 2009.