'archaeus' originates from Greek, specifically the word 'arkhaios', where 'arkhaios' meant 'ancient' or 'former'.

'archaeus' passed into Medieval and Renaissance Latin as 'archeus' (used by Paracelsus and other alchemists) and later entered English as 'archaeus' with specialized meanings in alchemy and natural history.

Initially it meant 'ancient' (from Greek), but in Paracelsan and alchemical usage it came to mean a 'vital principle' animating the body; in later scientific contexts the form was sometimes used to refer to members of the Archaea.