What does wave propagation mean in physics?

Let's start with propagation. The Merriam–Webster dictionary defines the word as a noun that means "the spreading of something." Wave propagation is therefore physics language for the movement or spreading of waves. Waves move energy outward from a source. The source could be just about anything. Light waves can be generated by stars, seismic waves by earthquakes, and sound waves by vibrating guitar strings. What is different about those waves is the direction of wave travel when compared to the direction of the wave vibration. Transverse waves have a vibration that is perpendicular to the direction of wave propagation. This is the kind that electromagnetic waves are. On the other hand, longitudinal waves have a vibration that is parallel to the direction of wave propagation. Sound waves are this type of wave and so are primary earthquake waves.
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