From The Collaborative International Dictionary of English v.0.48:

Ultragaseous \Ul`tra*gas"e*ous\, a. [Pref. ultra + gaseous.]
   Having the properties exhibited by gases under very low
   pressures (one millionth of an atmosphere or less). Matter
   under this condition, which has been termed the fourth state
   of matter, is sometimes called radiant matter.
   [Webster 1913 Suppl.]
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