Things that are intended for or understood by only a small group of people with specialized knowledge; secret or obscure matters.
From 'esoteric' (from Greek 'esoteros,' inner) + the Latin plural suffix '-a'. The word describes knowledge kept hidden from the general public, originally referring to secret philosophical teachings.
Medieval alchemy was full of esoterica—alchemists deliberately used cryptic symbols and secret recipes, partly for mystical reasons but partly to protect their discoveries from competitors and authorities. The tradition of insider knowledge creating mystery is ancient.
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