Luminiferous ether is a term that has been used to describe a hypothetical substance that was believed to fill all space and allow light to travel through it. However, in modern physics, the concept of luminiferous ether has been discredited. As a result, there are not many synonyms that have been used to describe this hypothetical substance. Some alternative terms that have been used in the past include "aether" or "etheric medium," but these terms are also no longer used in modern physics. Instead, physicists now use the theory of relativity to describe the behavior of light and other electromagnetic radiation.