"Speed of light" almost always means "speed in a vacuum", to a sufficient approximation speed between astronomical bodies. Density between local stars is around 1 proton per cubic kilometer, and it's much lower between galaxies.
So travel in media is excluded from the definition. What happens in media? There are two kinds of media; normal ones and specially prepared nonlinear ones.
In the normal media the photons travel until they are captured by an atom, then they are absorbed and the atom is driven by this to a higher energy state ("quantum leap"). After a moment it "relaxes" down to its prior state by emitting a new photon. Conservation of momentum means that the new photon has the same momentum as the old one, and it continues along the original path,... until it meets another atom and the process repeats. In this way the photons always travel at c (which, since they are massless, relativity requires them to do), but the time of passage, due to the successive atomic interactions, is longer and the effective speed from point to point is lower.
In the specially prepared nonlinear media, the group velocity and phase velocity are jiggered deliberately to make some wave form travel faster than the photons (held up by those interactions) can travel. That a wave form (but not a signal) can travel faster can be illustrated by swinging a flashlight while pointing it at a wall. Obviously the photons aren't traveling any faster, but if the wall was a light-year away, you could make the spot from the flashlight move faster than light. The clever experimenters can set up special media to make this happen in the laboratory, and reprters are always delighted to hear and report that "Einstein has been refuted".
