The property that describes an electromagnetic wave and not a mechanical wave is that electromagnetic waves can travel through a vacuum.

Here's why:

* Mechanical waves require a medium (like air, water, or a solid) to travel. They are caused by vibrations or disturbances within that medium.

* Electromagnetic waves are made up of oscillating electric and magnetic fields. These fields can exist and propagate even in the absence of matter.

Examples of electromagnetic waves include:

* Light

* Radio waves

* X-rays

* Microwaves

Let me know if you have any more questions!