According to the knowledge of the Ancients and Middle Ages, the fifth essence does not enter upon the terrestrial sphere in the matter of the universe in the space that it occupies; i.e., a very rarefied and highly elastic substance formerly believed to permeate all space including the interstices between the particles of matter, and to be the medium whose vibrations constituted light and other electromagnetic radiation; late Middle English: from Old French, or via Latin from Greek aithēr ‘upper air’, from the base of aithein ‘burn, shine’. Originally the word denoted a substance believed to occupy space beyond the sphere of the moon; ether (sense 3) arose in the mid 17th century and ether (sense 1) in the mid 18th century.