- a secret; a mystery
- specialized or mysterious knowledge, language, or information that is not accessible to the average person (generally used in the plural)
Example: Through the years, Usenet evolved into an international forum on thousands of topics, called Usenet news groups, from the arcana of programming languages to European travel tips. [Reference: Katie Hafner, "James T. Ellis, 45, a Developer of Internet Discussion Network, Is Dead", New York Times , July 1, 2001]
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