If you want to see a religion scholar twitch, try using the adjective 'mystical' as a catch-all descriptor for anything vaguely religious. The effect is about the same as you'd get if you asked a physicist to listen to a New Age devotee talk about 'energy.'There's probably even a small crossover population that you could drive into complete fits by putting the two together as 'mystical energies.'
Mysticism, like religion, is a term that's both problematic and necessary. It's necessary because it does seem to describe a category of experiences and practices that appear in many religions. It's problematic because that category is fuzzy enough that authors have to spend a great deal of time explaining and defending the specific definition that they're using before they can go on to talk about the subject.
I've generally used 'classic' scholars as my sources for these posts, so that's where I'll start to find a definition of mysticism. William James, in The Varieties of Religious Experience, provides the basic four-part definition that I'll paraphrase here. An experience can be called mystical, according to James, when it is:
Ineffable—The experience itself cannot be communicated completely. That's why mystical writings are generally obscure, poetic, or paradoxical in content: reading them, you can imagine the mystic having experienced a powerful something that's out of reach of language. It could be a moment of complete union with her god, a glimpse of the underlying structure of the cosmos, or a vision of the apocalypse, but the only way to talk about it is to talk around it.
Noetic—Even if the experience can't be communicated, the mystic still recognizes that it has imparted some sort of true knowledge. This doesn't necessarily have to be a tremendous revelation, either. The moment when someone 'gets' a religious idea, or sees the truth behind a familiar passage of scripture, could be considered a minor mystical experience in James' terms.
Transient—This isn't a mandatory part of the definition, but James suggests that it tends to be a feature of mystical experiences that they're brief. They could be instants of illumination or hours-long visions of the celestial hierarchy, but eventually, they come to an end and force the mystic to return to more mundane reality. There's been some debate about whether or not this particular point is valid: if a person's life and perceptions are permanently altered by the experience, has it really ended? It might be better to say that the mystic goes through more and less intense phases.
Passive—During the experience, the mystic may lose a sense of agency or even identity. Whatever happens, whatever is revealed, is beyond her control. That doesn't mean that there aren't practices that could make mystical states easier to attain; it means that once they begin, the mystic takes a passive role as the vessel of imparted knowledge.
These four features help distinguish mystical experiences from ordinary ones, but they don't do much to help you if you want to add a mystic as a character in a story, or make a mystical sect an element of the setting for a game.
"I am Marchosian the Mystic," the old man announced, "and am prone to having passive, transient experiences."
"That's nice," said Gert. "I have those, too. I call them 'naps.'"
"But mine impart knowledge!"
"And you wonder why more people don't talk to you."
Next time, I'll go into some common forms of mystical experiences, give a few real-world examples, and offer some suggestions on how you can use them for your stories.