A dense bio-energy that makes the materialisation of ghosts possible. The term ectoplasm was used in a similar sense in the 1984 film, Ghostbusters.
In Supernatural, ectoplasm appears as a viscous black liquid. It is typically excreted from either the tear ducts or ears of people being possessed by a ghost or left as a residue by an extremely powerful and angry spirit.
The spectre of Vance Collins leaks green ectoplasm all over the place when possessing people to kill people they have a grudge against. The green ectoplasm allows them to identify Vance as a spectre from Bobby's journal.