Strictly, the Theory of Special Relativity:

  1. applies to all observers who are moving at a constant speed,
  2. says that the laws of physics will apply equally to all those observers,
  3. says that the speed of light, if in a vacuum, will be the same for all those observers. (Remember this is what put paid to the ether theory: the fact that the speed of light was found to be unchanging in space showed that there was a vacuum and no ether.)