Arcanum, from the Latin arcere, "to shut away in an arca, cf. ark," is usually used to refer to occult mysteries and doctrines of various sorts. The plural of arcanum is arcana. In the oldest sense arcana were the rituals and artifacts of mystery religions and secret societies, which were not supposed to be shown or disclosed to outsiders. The word has a number of uses.

• In alchemy, the Grand Arcanum describes the secret of the philosopher's stone;
• By extension, in one older sense an arcanum is used for any nostrum said to be a panacea, such as the philosopher's stone was believed to be;
• Occult interpretations of the Tarot cards divide the deck into the Major or Greater Arcana, the trumps, and the Minor or Lesser Arcana, the numbered pip cards.
• Arcanum: Of Steamworks and Magick Obscura is also the title of a computer role-playing game by Troika Games.
• A pen and paper role-playing game by Bard Games based in the Lost Word of Atlantis.
• Arcanum is a village located in Darke County, Ohio.
• Arcana Studio is also the name of a popular comic book company.