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Rarefaction is the reduction of a medium's density, or the opposite of compression.

A natural example of this is as a phase in a sound wave or phonon. Half of a sound wave is made up of the compression of the medium, and the other half is the decompression or rarefaction of the medium.

Another natural example of rarefaction is in the layers of our atmosphere. Because what constitutes our atmosphere has mass, it is definite that most of the atmospheric matter will be nearer to the Earth. Therefore, air at higher layers of the atmosphere is considered to have less pressure, or is rarefied in relation to air at lower layers.

Rarefaction can be easily observed by compressing a spring and releasing it.

Rarefaction waves expand with time; for most gases the rarefaction wave keeps the same overall profile at all times (it is a 'self-similar expansion'). Each part of the wave travels at the local speed of sound, in the local medium. This expansion behaviour is in contrast to the behaviour of pressure increases, which get narrower with time, until they steepen into shock waves.

Rarefaction can refer to an area of low relative pressure following a shockwave.

The term rarefaction in ecology refers to a technique to standardize and compare species richness computed from samples of different sizes. Rarefaction allows the calculation of the species richness for a given number of sampled individuals and allows the construction of so called rarefaction curves. This curve is a plot of the number of species as a function of the number of individuals sampled. In case of a steep slope a large fraction of the species diversity is not sampled, if the part of the curve is already becoming flat, a reasonable number of individuals is sampled and more intensive sampling will probably only yield a small number of additional species (if any). [1]

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