Originally this name comes from the old greek 'ousía' meaning 'essence' or 'substance'. What is actually this old greek 'substance'? Pretty much something that makes a certain being unique, it's an ontic core of something living. Official deifnition says it's a being's internal principle of operation and organisation. According to greek philosophy, only living beings have substance; if I would risk extrapolating philosophical method on mathematics, a 'substance' would most likely be some sort of algorithmic system organizing whole structure of the being, it's like an operating system of things in the reality, a law to be precise. Anyway - the subtlety and elusiveness of incorporeal property of these organisms suggest that their name should be connected to something... at least as peculiar as sole fact, that these beings, these wraiths are somehow living as - just guessing - self sustaining systemic wave-particle duality organism.