Ectoplasm (from Greek ektos "outside" and plasma "something molded") is a type of substance, or spiritual energy, often associated with the appearance of ghosts and acts of a psychic nature. How it is actually made, what it is made of, or if it's even actual matter (e.g. atoms) are all questions which no person as been able to answer as of yet. Any public testing done on purported samples of ectoplasm have all turned up negative, including the effects of ectoplasm in photography. Due to its mysterious nature ectoplasm as grown very popular in western culture, appearing in the realms of fantasy games and television shows on the supernatural (i.e. the actual show Supernatural). The only general consensus is that ectoplasm is some kind of sticky liquid like substance that looks like it's full of white, dead skin. This of course leads to interesting, if not disturbing, images of what people do in the lonely places one would often find ghosts.
All joking aside, ectoplasm still remains one of the most mysterious and confusing aspects of the paranormal, which quite frankly is saying a lot.