Plasm (sometimes called ectoplasm) is a metaphysical substance featured in Wraith: The Oblivion. Plasm is the generic "stuff" out of which most wraithly material and the Underworld itself is manufactured. The "bodies" of disembodied entities, including wraiths, Spectres and Plasmics are composed of plasm. Plasm is not a substance in the physical sense; lacking atoms, molecules or a dipole moment, but it is close enough for soulforging work.
Since Wraith is not science fiction, the actual behavior and composition of plasm (even, to a certain extent, what parts of the underworld are plasm) is open to debate. However, at the minimum, there is both liquid and solid plasm - the liquid is used to temper items made with soulforging, and the solid is everything else. In general, the only source of plasm is the bodies of wraiths; any artifact in the wraithly world has been made of plasm, and consequently is made out of a wraith.