Constancy principle (A. Einstein): One of the postulates of Sir Einstein's special theory of relativity that puts forth that the speed of light in vacuum is computed as similar speed to all observers, in spite of of their relative motion. That is, if I'm travelling at 0.9 c away from you, and fire a beam of light in that direction, both you and I will separately compute the speed of that beam as c.
One of the outcomes of this postulate (i.e., one of the predictions of the special relativity) is that no massive particle can be accelerated to (or beyond) the light speed, and therefore the speed of light also symbolizes the eventual cosmic speed limit. Only mass-less particles (collectively termed as luxons, comprising photons, gravitons, and probably neutrinos, must they prove to indeed be mass-less) travel at light speed, and all other particles should travel at slower speeds.