According to the theory of special relativity, the speed of light in a vacuum, denoted by "c," is considered to be constant and independent of the motion of the source or observer. This principle is known as the "constancy of the speed of light."
No matter how fast an observer or a light source is moving relative to each other, the measured speed of light will always be the same. This is a fundamental postulate of special relativity and has been extensively validated by numerous experiments and observations.
This constancy of the speed of light leads to some counterintuitive effects, such as time dilation and length contraction, as described by the theory of relativity. These effects occur as a result of the relative motion between observers and the differences in how time and space are experienced by them.
In summary, regardless of the motion of the source or observer, the speed of light in a vacuum remains constant at approximately 299,792 kilometers per second.