Final answer:
The substance believed to emanate from a medium during a seance is called ectoplasm. It is a term used in spiritualism to describe a spiritual energy or substance produced by a medium.
Step-by-step explanation:
The substance that some believe emanates from a medium during a seance is known as ectoplasm. Ectoplasm is a term used in spiritualism to describe a substance or spiritual energy that is exuded by a medium during a seance, thought to be materialized by spirits. The concept of ether as a medium that carried light waves is unrelated and was a scientific theory proposed to explain the propagation of light before the concept was disproven by experiments. Ectoplasm is often depicted as a viscous, gelatinous substance that is said to be produced by mediums when they are in a trance and in communication with the spirit world.