Presumably at those speeds, the frame of reference would be the galaxy and the interstellar material in which the ship travels. If your ship were to match the a

Presumably at those speeds, the frame of reference would be the galaxy and the interstellar material in which the ship travels. If your ship were to match the average speed of this material, then you could claim to be at a stop. If the concern was to prevent damage due to collision with this material, then it makes sense that in order to "stop" you would actually have to match the speed of the material that you are moving through.