What is the scientific definition of rare fractions?

Rarefaction is a scientific term that describes a reduction in density and pressure in a medium, typically related to waves, such as sound waves. In a rarefaction phase, particles in a medium, like air or water, are spread farther apart than in their normal state, leading to a decrease in the medium's density. This phenomenon occurs during the passage of a longitudinal wave, where regions of compression alternate with regions of rarefaction.