Here's why:

* Periodic: This means the disturbance repeats itself at regular intervals. This creates the characteristic pattern of crests and troughs or compressions and rarefactions that we associate with waves.

* Disturbance: This means there is a change in the medium, whether it's a displacement of particles (like in a water wave) or a change in pressure (like in a sound wave).

* Medium: This is the material through which the disturbance travels. Waves can travel through various media, including solids, liquids, gases, and even the electromagnetic field (like light).

Key points:

* Waves transfer energy, not matter: While the disturbance travels through the medium, the particles themselves don't move along with the wave. They oscillate or vibrate around their equilibrium position.

* Types of waves: There are different types of waves categorized by their direction of oscillation relative to their direction of travel (transverse and longitudinal).

In summary: A periodic disturbance that propagates through a medium is the fundamental definition of a wave.