For the Christian the best definition I have ever read is, 'The entrance into the mystery of Christ'. So when we have receive the Sacraments , for instance we experience the Mystical. The Eucharist is the best example of all. People when they think of the mystical always think of things like visions. But still waters run deep. The fact that a river runs deep under the ground does not make it any less a river. There could be a whole ocean below us and we not know. The Eucharist is so like this. Whole oceans of graces, hidden.