(1) Compression:
A compression is a region in a longitudinal wave where the particles are closest together.
(2) Rarefaction:
A rarefaction is a region in a longitudinal wave where the particles are furthest apart.
(3) Medium
The material or empty space through which signals, waves or forces pass.
(4) Vacuum:
Vacuum, is a region without any matter, or a region of zero pressure (perfect vacuum). It is practically impossible to create perfect vacuum. So real vacuum should actually be defined as a region almost devoid of matter, or of very low pressure.
