Æther

Æther, historically, was a concept in physics suggesting a mysterious, all-pervasive medium filling space, believed to be the carrier of light and electromagnetic waves. It was thought to be invisible, weightless, and elastic, enabling interactions over distances. The idea was prominent before the development of Einstein's theory of relativity, which showed that space itself can bend and that light doesn't need a medium to travel. Today, æther is considered an outdated concept, replaced by modern physics' understanding of space, quantum fields, and the fabric of spacetime.