arcane

adjective4

1. •
   Understood by only a few.

   • The professor’s lecture was full of arcane references.
   • The manuscript contained arcane symbols no one could decipher.
2. •
   Obscure, mysterious.

   • arcane details
3. •
   Requiring secret or mysterious knowledge to understand.
4. •
   Extremely old (e.g. interpretation or knowledge), and possibly irrelevant.