According to the theory of special relativity, the speed of light in a vacuum, denoted by "c," is constant for all observers, regardless of their relative velocity or direction. This principle is known as the "constancy of the speed of light."
One of the key postulates of special relativity is that the laws of physics are the same in all inertial reference frames. An inertial reference frame is a coordinate system in which an observer at rest or moving with a constant velocity experiences no acceleration.
In this framework, the speed of light is considered to be a fundamental constant of nature, and it is the same for all observers in all inertial reference frames. No matter how fast or in which direction an observer is moving, they will measure the speed of light to be the same value, approximately 299,792,458 meters per second (or about 186,282 miles per second) in a vacuum.
However, it's important to note that the measured passage of time and the lengths of objects can appear to change for observers who are moving relative to each other. This phenomenon is known as time dilation and length contraction, respectively, and it is a consequence of the principles of special relativity.