Rarefaction

noun

1. The condition of being rarefied.
3. A decrease in density and pressure in a medium, such as air, caused by the passage of a sound wave.
4. The region in which this occurs.
5. The act or process of rarefying or making rare, or of expanding or distending a body or mass of matter, whereby the bulk is increased, or a smaller number of its particles occupy the same space; also, the state or condition so produced: opposed to condensation.