The type of waves where energy travels perpendicular to the motion of the medium are called transverse waves.

Here's why:

* Transverse waves: The particles in the medium oscillate perpendicular (at right angles) to the direction the wave travels. Think of a rope tied to a post, if you shake the rope up and down, the wave travels horizontally along the rope, while the rope itself moves up and down.

* Longitudinal waves: The particles in the medium oscillate parallel to the direction the wave travels. A good example is sound waves. The molecules in the air compress and expand in the same direction the sound wave is moving.

In summary:

* Transverse waves: Energy travels perpendicular to the medium's motion.

* Longitudinal waves: Energy travels parallel to the medium's motion.