Ectoplasm

An Ectoplasm is an energy field that surrounds a ghost, giving it some solidity or "skin."

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 demon Troxa while invisible was said to have ectoplasmic biochemistry, which could be made visible with cold temperatures. ("Sight Unseen")