What Is Mysticism?

The Oxford English Dictionary defines mysticism as the belief in the possibility of union with the divine. The problem here is that the definition seems to want to describe mysticism as a form of belief, a kind of theoretical possibility. It's surely such a belief implies that you believe also that people have attained to this union, or at least could in principle? So if the belief is mysticism, what about the actual attainers, the practitioners of mysticism one? Are they perhaps to be defined as something like practical mystics, but what they're doing can't be defined as mysticism because that definition is already taken to describe the people who believe in them?