The speed of an electromagnetic wave in a vacuum is approximately 299,792,458 meters per second (m/s). This is often rounded to 3 x 10^8 m/s.

Here are some key points:

* Constant Speed: Electromagnetic waves, including light, travel at this constant speed in a vacuum, regardless of their frequency or wavelength.

* Speed of Light: This speed is often referred to as the "speed of light" because visible light is part of the electromagnetic spectrum.

* Medium Dependence: The speed of an electromagnetic wave can change when it travels through a medium other than a vacuum. For example, light travels slower in water or glass.

Let me know if you have any other questions!