Definitions of mysterium:
- noun: (chemistry, alchemy, now historical) Any of various unknown elements thought to make up existing forms of matter, or a substance seen as an elemental or pure form of something else.
- noun: (astronomy, now historical) The hypothetical source of a galactic radio emission at 1665 megahertz (later identified as due to hydroxyl radicals in interstellar space).
(Definitions from Wiktionary)
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