RAREFACTION

rarefaction

(noun) a decrease in the density of something; “a sound wave causes periodic rarefactions in its medium”

Source: WordNet® 3.1

Etymology

Noun

rarefaction (countable and uncountable, plural rarefactions)

A reduction in the density of a material, especially that of a fluid.

Antonyms

• compression

Source: Wiktionary

Rar`e*fac"tion, n. Etym: [Cf. F. raréfaction. See Rarefy.]

Definition: The act or process of rarefying; the state of being rarefied; - - opposed to condensation; as, the rarefaction of air.

Source: Webster’s Unabridged Dictionary 1913 Edition