What is ether in huygen's wave theory of light?

In Huygens' wave theory of light, ether (or "luminiferous ether") was a hypothetical medium through which light waves were thought to propagate. Huygens proposed that every point on a wavefront serves as a source of secondary wavelets, which spread out in all directions, creating the wave's propagation through this ether. Although ether was a widely accepted concept in the 19th century, it was later discarded after experiments, such as the Michelson-Morley experiment, failed to detect it, leading to the development of the theory of relativity.