A secret society is an organization about which the activities, events, inner functioning, or membership are concealed. The society may or may not attempt to conceal its existence. The term usually excludes covert groups, such as intelligence agencies or guerrilla warfare insurgencies, that hide their activities and memberships but maintain a public presence.
"Secret Society Buildings at Yale College" by Alice Donlevy c. 1880. Pictured are: Psi Upsilon (Beta chapter), 120 High Street. Left center: Skull and Bones (Russell Trust Association), 64 High Street. Right center: Delta Kappa Epsilon (Phi chapter), east side of York Street, south of Elm Street. Bottom: Scroll and Key (Kingsley Trust SSS Nonse Association), 490 College Street.
The Brethren of Sincerity) were a secret society of Muslim philosophers in Basra, Iraq, in the 9th or 10th century CE.
A Hongmen seal, 19th century
Secrecy is the practice of hiding information from certain individuals or groups who do not have the "need to know", perhaps while sharing it with other individuals. That which is kept hidden is known as the secret.
U.S. soldier at camp during World War II.
Loose lips might sink ships, World War II slogan
First secret confidence to Venus, François Jouffroy
A Secret from on High (Secret d'en haut), Hippolyte Moulin (Hypolite Moulin), 1879