The velocity of an electromagnetic wave in a vacuum is a fundamental constant known as the speed of light, denoted by the letter c.

Here's the breakdown:

* c = 299,792,458 meters per second (m/s)

* This value is incredibly fast! It takes light approximately 8 minutes and 20 seconds to travel from the Sun to Earth.

Important Notes:

* In different mediums: The speed of light changes when it travels through different mediums like air, water, or glass. This is because the wave interacts with the atoms of the medium, slowing it down.

* The speed of light is the fastest speed possible in the universe. No object or information can travel faster than the speed of light.

Let me know if you'd like to learn more about the properties of electromagnetic waves or how they interact with matter!