Physicists tend to be very pragmatic folks when it comes to certain things.
The speed of light was measured by Danish astronomer Ole Roemer back in 1675. He noticed that the predicted transits of Jupiter by its satellites would come over an hour after they were scheduled, and concluded that this was because light took time to get from Jupiter to earth. SInce the distance to Jupiter was known, it was a simple matter to divide this by the delay time to get the velocity of light pretty accurately. Today, thanks to high speed electronics, you can measure time intervals to nanoseconds, during which time light travels about a foot or so. As for what light is of itself, the answer depends on whether you which to speak the language ot particles or waves.