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.
In 2.06 No Exit, ectoplasm is generated by the spirit of Dr. H.H. Holmes. Dialogue between Sam and Dean indicate that the spirit would need to be extremely powerful (and pissed off) in order to create ectoplasm.
In 8.06 Southern Comfort, 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.