Ectoplasm was coined from the Greek words ektos and plasma, which together means "outside formed." Ectoplasm is used to describe spiritual energy with substance. Charles Richet created the word ectoplasm to first describe physical manifestations that appeared from psychic mediums during the late 1800's while in trances. Ectoplasm was purportedly secreted from the medium's orifices, such as the ears, nose or mouth. However, ectoplasm of this type was more of a hoax during the rise of spiritualism and fraudulent mediumship.
Today, ectoplasm has come to represent physical manifestations of mists believed to be ghosts. At first, being something more of legend and lore, the idea of 
ectoplasm was still popular, and eventually was used in plays, movies and books. But ectoplasm had a real rise in cultural popularity after the comedic movie Ghost Busters described green slime emitted by ghosts as "ectoplasmic residue." Most everyone who has ever seen the film can remember people getting "slimed" by the ghosts.
In modern times, ectoplasm in ghost investigations may still be described as "any unknown physical substance," such as a slime, attributed to a haunting; but, mostly, photographic and video evidence of mists and vapors captured during ghost investigations are being labeled ectoplasm. When a mist is present that cannot be attributed to moisture, fog, smoke, cigarette smoke, steam, or breathe in cold air, the vaporous cloud can usually be assumed to be a physical manifestation of spirit.
Ectoplasm can be called "ghost mists," "ghost fog," or "ghost vapors" by paranormal investigators. Ectoplasm has been described as the "precursor" to the formation of an apparition; the mist, sometimes, being witnessed first. Then, the ectoplasmic formation may take on a human shape with details, including body features, color, clothing, etc.
More Old-time Ectoplasm Info:
Ectoplasm & Mediums
Spirit Photography
See Real Ectoplasmic Mists:
Ectoplasm Ghost Pictures
Ectoplasm Ghost Photos
Mists & Vapors Ghost Pics
Compare:
False Ectoplasm Pictures