A wave that vibrates in a direction parallel to the direction of its travel is called a longitudinal wave.

Here's why:

* Longitudinal waves: The particles in the medium vibrate back and forth in the same direction as the wave is traveling. Think of a slinky being stretched and then pushed, creating a compression wave that travels along the slinky. Sound waves are also longitudinal waves.

* Transverse waves: The particles in the medium vibrate perpendicular to the direction the wave is traveling. Imagine shaking a rope up and down, creating a wave that travels along the rope. Light waves are transverse waves.