rarefaction

A decrease in the density of something is rarefaction. As you climb a mountain, you experience rarefaction of the air; the air becomes less dense the higher up you go.

Most of the time, rarefaction refers to air or other gases becoming less dense. When rarefaction occurs, the particles in a gas become more spread out. You may come across this word in the context of sound waves. A sound wave moving through air is made up of alternating areas of higher and lower density. The areas of lower density are called rarefactions.

Definitions of rarefaction
1

n a decrease in the density of something

“a sound wave causes periodic rarefactions in its medium”
Type of:
concentration
increase in density

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