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.