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From The Collaborative International Dictionary of English v.0.48:
Ultragaseous \Ul`tra*gas"e*ous\, a. [Pref. ultra + gaseous.] (Physics) 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.]