Definition of "arcanum" []

• A deep secret; a mystery. (noun)
• Specialized knowledge or detail that is mysterious to the average person: "knows the arcana of police procedure and the intricacies of litigation” ( George F. Will). (noun)
• A secret essence or remedy; an elixir. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "I see exactly what you mean, and it fits really well with some of my own vaguer concerns with the belief in arcanum that marks out magic realism."
• "In fact, it's precisely because we do not know how it works that we do not rule out the possibility that it does in fact work, the possibility even that others might understand the workings we do not -- hence the term arcanum, with its associations of secret knowledge."
• "Freemasonry were off-shoots of the original Cabala, and that during the past 150 years new associations had been formed, and the parties who had introduced me into their arcanum were a society in affiliation with many others then in existence in different countries."