The region of a longitudinal wave in which density and pressure are less than normal is called a rarefaction.

Here's why:

* Longitudinal Waves: These waves travel by compressing and expanding the medium they travel through. Think of a spring: you compress it (high density) and then release it (low density).

* Compression: The area where the medium is compressed is denser and has higher pressure than the undisturbed medium.

* Rarefaction: The area where the medium is expanded is less dense and has lower pressure than the undisturbed medium.

In summary: Rarefactions are the "thin" spots in a longitudinal wave, where the medium is less dense and the pressure is lower.