rarefaction

Noun

rarefaction (countable and uncountable, plural rarefactions)

1. A reduction in the density of a material, especially that of a fluid.
   • [T]his ſeal'd vial, being broken under water, ſuck'd in a conſiderable quantity of it, whether, becauſe of ſome little rarefaction of the Air included in the ſealing, or becauſe of the infrigidation of that Air by the ſnow, or for both theſe Reaſons, or any other, I ſhall not Now diſpute.1663, Robert Boyle, “Title I. Experiments Touching Bodies Capable of Freezing Others.”, in New Experiments and Observations Touching Cold, or, An Experimental History of Cold, Begun. […], London: […]...
   • ..; but be particular to keep a firm hold over the cork of the bottle, or else the rarefaction will blow the cork out with the force of a shot: and an accident might acrue from fire.1847, editorial staff, “White Hard Spirit of Wine Varnish”, in Scientific American, series 1, Volume 3, Issue 9, page 72