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Alternative forms[edit]


From Middle English rarefien, from Old French rarefier (French raréfier), from Latin rarefacere (make rare). By surface analysis, rare +‎ -fy.


  • IPA(key): /ˈɹɛəɹəˌfaɪ/
  • (file)


rarefy (third-person singular simple present rarefies, present participle rarefying, simple past and past participle rarefied)

  1. (transitive) To make rare, thin, porous, or less dense.
    • 1815, A Philosophical and Mathematical Dictionary: Containing an Explanation of the Terms, and an Account ... by Charles Hutton ... vol. 1[-2], page 56:
      By means of this gauge, Mr. Smeaton judged that his machine was incomparably better than any former one, as it seemed to rarefy the air in the receiver 1000, or even 2000 times, while the best of these only rarefied it about 140 []
  2. (intransitive) To become rare, thin or less dense.
    • 1916, Wisconsin Geological and Natural History Survey, Bulletin, page 293:
      For if The[sic] water rarefies and then Condenses, all that great mass of water of the Lake of the Ilinois rises in its vast basin when it rarefies, and falls when it condenses. And, as water always rises as much as it falls, it would []
    • 2009, John C. Malone, Psychology: Pythagoras to Present, MIT Press, →ISBN, page 28:
      As water rarefies, it evaporates, forming visible steam, and this may be a step toward the final category of reality, fire. Anyone who has held a hand over a boiling kettle may feel the truth of this. []
    • 2019 November 29, Peta Morton, Ancient Teachings for Modern Times: The Way To a Rich and Deeply Satisfying Life, John Hunt Publishing, →ISBN:
      Given the high quantity of water molecules in the human body it is worthy of note at this point that moisture rarefies when heated. []