When the particles of a medium are far apart, that part of the wave is called a rarefaction.

Here's why:

* Waves in a medium: Waves travel through a medium by causing the particles of that medium to vibrate.

* Compression: When the particles are close together, it's called a compression.

* Rarefaction: When the particles are spread out, it's called a rarefaction.

This concept is particularly relevant to longitudinal waves, like sound waves, where the vibrations occur parallel to the direction of wave propagation.