In the theory of special relativity, the speed of light in a vacuum, denoted by the symbol 'c,' is considered an absolute constant. It is the same for all observers, regardless of their relative motion. According to this theory, the speed of light is not relative to anything specific within the framework of special relativity.
The key principle of special relativity, known as the principle of relativity, states that the laws of physics should be the same for all observers in uniform motion relative to each other. No matter how fast one observer is moving relative to another, they will always measure the speed of light in a vacuum to be 'c.'
This principle leads to some intriguing and counterintuitive consequences, such as time dilation and length contraction, as well as the equivalence of mass and energy (E=mc²). It also implies that as an object with mass approaches the speed of light, its energy requirements to accelerate further become infinite, making it impossible to reach or exceed the speed of light.
So, to summarize, the speed of light is an absolute constant in special relativity and is not relative to any particular observer or reference frame.