The speed of an electromagnetic wave in a vacuum is a fundamental constant in physics and is denoted by the letter c. Its value is approximately:

c = 299,792,458 meters per second

This speed is often rounded to 3 x 10^8 m/s for convenience.

Here's why this speed is so important:

* It's the fastest speed anything can travel in the universe. This is a consequence of Einstein's theory of special relativity.

* All electromagnetic waves travel at this speed in a vacuum, regardless of their frequency or wavelength. This includes light, radio waves, X-rays, and gamma rays.

* This speed is used to define the meter. The meter is defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second.

Let me know if you have any other questions!