The part of a longitudinal wave that is least dense is called a (n)

rarefaction

refraction

contraction

compression

Rarefraction

The part of a longitudinal wave that is least dense is called rarefraction. Rarefraction is the reduction of an item's density, the opposite of compression. Like compression, which can travel in sound waves for instance, rarefaction waves also exist in nature. A common rarefaction wave is the area of low relative pressure following a shock wave.