A profound secret or mystery, or a secret remedy or formula known only to a few people; plural arcan.
From Latin 'arcanum' (secret thing), derived from 'arcanus' (secret, closed up), which may relate to 'arcere' (to shut up, enclose), suggesting something hidden away or kept enclosed from view.
Medieval alchemists used 'arcanum' to describe the secret formulas they believed would turn lead into gold—today the word perfectly captures the mystery of hidden knowledge, like cryptography or classified research!
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