I’m risking over simplifying things, but in Catholicism, mysticism is really just a way of saying “an experience of God”. A mystical experience could run the gamut from getting a little choked up during communion or while gazing upon a crucifix, or it could include total ecstatic rapture, healings, or more unusual things like physically floating away (St. Cupertino) and stigmata (St Rita, Padre Pio etc) and the kind of contemplation and union with God that Saint Theresa talked about. It could also include visions or apparitions of Jesus or Mary.
My definition of a mystic is someone whose spirituality includes some sort of “experience” or “encounter” and for whom these experiences have a profound enough impact fit into the core of their spirituality and understanding of God.