Definition: UPG or "Unverified Personal Gnosis" is a term used to describe subjective magical experiences that matter to the individual but that either mean something else or nothing at all to others.
Another way to think about it: every individual has a subjective universe that is made up of his or her limited perceptions, beliefs, and assumptions—including magical thinking. The objective universe is the world beyond each individual subjective universe, a world only partly understood and imperfectly perceived. UPG is part of an individual's subjective universe and may work for that person, but it does not necessarily transcend into the objective universe or the subjective universes of others. Think of UPG like a personal set of glasses through which a magical practitioner views experience.
A Helpful Alternative: The key question to ask is not whether something is "true" or "real" (there are all sorts of beliefs regarding what is "really true" and "really real"). Rather, one should ask, "Is this useful?"