Constancy principle (A. Einstein):
One of the postulates of A. Einstein's special theory of relativity, that puts forth that the speed of light in vacuum is calculated as the same speed to all observers, in spite of of their relative motion. That is, if I'm travelling at 0.9 c away from you, & fire a beam of light in that direction, both you & I will measure independently the speed of that beam as c.
A result of this postulate (one of the predictions of special relativity) is that no massive particle can be accelerated to (or beyond) lightspeed, and therefore the speed of light also represents the critical cosmic speed limit. Only massless particles (jointly called luxons, including gravitons, photons and possibly neutrinos, must they prove to certainly be massless) travel at lightspeed, and all other particles have to travel at slower speed