rarefaction
/ˌreərɪˈfækʃən/noun
mass noun
1- diminution in the density of something, especially air or a gas（尤指空气、气体的）稀薄化, 稀薄状态。
1.1
- Medicine the lessening of density of tissue, especially of nervous tissue or bone【医】（尤指神经组织、骨质的）稀疏（状态）, 疏松（状态）。
词源
early 17th cent.: from medieval Latin rarefactio（n-）, from the verb rarefacere 'grow thin, become rare'.