Etymological meaning: (mystic) Middle English mistik, from Latin mysticus of mysteries, from Greek mystikos, from mystēs initiate

Broad meaning (primary definition): a religious tendency and desire of the human soul towards an intimate union with the divinity, or a system growing out of such a tendency and desire

Strict theological meaning:  that which pertains to the operation of God in the soul which only He can accomplish and which man can in no way initiate; this primarily refers to those gifts bestowed for the benefit of the individual’s sanctification, and is set opposite and above asceticism, which is man’s own effort, assisted by grace, to act virtuously; it also includes the narrow sense below

Narrow sense: that which pertains to extraordinary gifts of God such as ecstasies, visions, levitation, private revelations, which are bestowed for the edification of the Church, rather than that of an individual’s sanctification, and which neither presupposes nor brings about holiness

The relationships between these different meanings and their nuances can be found in my first post on mysticism.