From Studies Of The Paranormal.com:
A strange, mystical material that flows out of any orifice and opening of a mediums body during a seance. This paranormal substance would sometimes take the shapes of faces, limbs, or even an entire figure but could not materialize until it had come out of the body of the medium. The typical color of ectoplasm is a creamy off-white. Quite often this substance was warm and life like but has also been known to be cold. This paranormal material could be doughy, sticky, airy, smoky, or thick and syrupy. This ghostly substance was also known to smell like ozone. Ectoplasm often seemed to form around believers of the paranormal and open minded, and not so much in the presence of the skeptical. Ectoplasm would form most of the time in the dark or in dim lighting. In bright light it would vanish completely or snap back violently.
Note from the editor: Scroll down the web page above to see a picture of "a medium named Mina Stinson Crandon producing ectoplasm in seance."
The designation of ectoplasm as a physical manifestation of the spiritual power or presence of a ghost is traced back to Charles Richet. While not understood to have a cell structure that is identical to human cells, ectoplasm is nevertheless considered to possess a small degree of weight and can be touched. Over the years, many persons have claimed to have come in contact with ectoplasm, and in fact have captured a portion of the purported supernatural material for study and analysis.