This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. A decrease in density and pressure in a medium, such as air, caused by the passage of a sound wave.

n. The region in which this occurs.

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

n. The act or process of rarefying; the state of being rarefied; -- opposed to condensation.

n. 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.

n. a decrease in the density of something

"But if, for example, the rise in the emulsion to double the rarefaction is a milliard times less than in oxygen, it means that the effective weight of the grain is a milliard times greater than that of the oxygen molecule."

compression   

concentration   

pithiness    volatilization    ancientry    intumescence    pressure