- the act or process of making less dense or the state of being less dense
c.1600, from Middle French raréfaction or directly from Medieval Latin rarefactionem (nominative rarefactio), noun of action from past participle stem of Latin rarefacere (see rarefy). Used chiefly in reference to gasses.
- A decrease in density and pressure in a medium, such as air, caused by the passage of a sound wave.
- A decrease in density and pressure in a medium, such as air, especially when caused by the passage of a wave, such as a sound wave.
- The region in which this occurs.