rarefaction
noun
rar·e·fac·tion ˌrer-ə-ˈfak-shən 
1
: the action or process of rarefying
2
: the quality or state of being rarefied
3
: a state or region of minimum pressure in a medium traversed by compressional waves (such as sound waves)
rarefactional
ˌrer-ə-ˈfak-shnəl
adjective -shə-nᵊl
Example Sentences
Recent Examples on the Web For rarity, for sheer automotive rarefaction, the flagship of BMW’s 2 billion-euro i Division makes those cars look like link sausage. 
Dan Neil, WSJ, 31 Aug. 2017 Here is art that turns confectionery into both rarefaction and panache. Alastair Macaulay, New York Times, 19 May 2017
Dan Neil, WSJ, 31 Aug. 2017 Here is art that turns confectionery into both rarefaction and panache. Alastair Macaulay, New York Times, 19 May 2017 Word History
Etymology
French or Medieval Latin; French raréfaction, from Medieval Latin rarefaction-, rarefactio, from Latin rarefacere to rarefy
First Known Use
1572, in the meaning defined at sense 1
Medical Definition
rarefaction
noun
rar·e·fac·tion ˌrar-ə-ˈfak-shən, ˌrer-
1
: the action or process of rarefying
2
: the quality or state of being rarefied
especially : an increase in porosity
areas of rarefaction in the bones
3
: a state or region of minimum pressure in a medium transversed by compressional waves (as sound waves)
the eardrum vibrates according to the compressions and rarefactions of the sound wave