In shear waves (also known as S-waves), the particle motion is perpendicular to the direction of energy propagation.

Here's a breakdown:

* Energy propagation: This is the direction the wave travels, like a ripple on a pond.

* Particle motion: This refers to how the individual particles of the medium are moving. In a shear wave, these particles move back and forth *perpendicular* to the wave's path.

Think of it like shaking a rope up and down. The wave travels along the rope (energy propagation), but the rope itself moves up and down (particle motion).

This is different from compression waves (like sound waves), where the particle motion is parallel to the direction of energy propagation.