mysterium

plural mysteria

1. historicalalchemyAny 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.

   There are many such mysteria: milk is a mysterium of cheese and butter, and cheese in turn a mysterium of maggots, which were thought to form spontaneously in rotting food.

   2006, Philip Ball, The Devil's Doctor, Arrow, published 2007, page 263

2. historicalastronomyThe hypothetical source of a galactic radio emission at 1665 megahertz (later identified as due to hydroxyl radicals in interstellar space).