[b3orn]

It's c, but not the same c as in air or vacuum. The same applies in optic fibers. They're both around two thirds of the speed of light in vacuum.

[GuB-42]

c is constant, the speed of light is not.

c is the speed of light in a vacuum, but it is not really about light, it is a property of spacetime itself, and light just happens to be carried by a massless particle, which, according to Einstein's equations, make it go at c (when undisturbed by the medium). Gravity also goes at c.

[bigfishrunning]

I've always considered C the speed of light and gravity goes at the speed of light, not that light and gravity both go C, which is a property of spacetime. This is a much simpler mental model, thanks for the simple explanation!

[Sesse__]

c is the speed of light in vacuum.

EM signals move at about 0,66c in fiber, and about 0,98c in copper.

[BenjiWiebe]

More like 0.6c to 0.75c in Cat6 Ethernet cable.

The insulation slows it down.