Ectoplasm has a few definitons, but is often described the supposed visual representation or physical manifestation of emanations from psychic mediums, magic wielders, and other paranormal characters during seances, spell castings, and other paranormal encounters as a type of light, or a moving mist. 
Other definitions describe Ectoplasm as the "aura" which surrounds a ghost or other paranormal spirit, and lastly, Ectoplasm can take form as a type of hard construct, appearing in different forms which can affect spirits and humans, most commonly as a gooey substance.