A magical substance or energy from the Spiritual Plane
Ectoplasm allows spirits to form in the physical plane
Ectoplasm, also referred to as Plasma, is a substance or spiritual energy field that surrounds spirits when appearing in the physical plane. Although mostly associated with spirits, certain Demons also use ectoplasm and some spirits can create offensive Plasma Balls with it.
When Phoebe and Prue were helping Andy Trudeau investigate several murders linked to a ghost from Alcatraz, he brought them a knife that had a fingerprint that only showed up under ultraviolet light. They informed him that ultraviolet fingerprints are what ghosts leave behind. ("The Power of Two")
The ghost Olivia Callaway could form spheres of ectoplasmic energy, known as Plasma Balls. She was the only ghost who was shown able to conjure them, possibly due to possessing the ability to throw Energy Balls when she was alive. ("Love's a Witch")