The nature of time...

[Kraxis]

Actually the speed of light varies depending on the substance it passes though, or the lack of it of course. In water for instance it is very very slow compared to our general image of 300,000 km/s. Even passing through our atmosphere it slows down considerably. This shows a relation to mass, despite the fact that light isn't a particle.
But in general we consider the speed of light to be the one in absolute vacuum.

[_Martyr_]

Eh, read what I wrote... I did indeed mention vacuum. Why else would refraction exist?

[Kraxis]

We wrote it at the same time... I was answering Quietus.

[Quietus]

We wrote it at the same time... I was answering Quietus.

Yes, speed of light is a given depending on the medium.