According to philosopher Jerome Gellman, a mystical experience is a “super sense-perceptual or sub sense-perceptual experience granting acquaintance of realities or states of affairs that are of a kind not accessible by way of sense perception, somatosensory modalities, or standard introspection” (1). 

1. By “super sense-perceptual” one means perception-like content that is not sense perception, somatosensory modalities (such as the sensing of pain and the internal sensing of the body, limb, organ, etc.), or standard introspection.
   A sub sense-perceptual experience has no phenomenological content although such an experience can also consist of phenomenological content appropriate to sense perception, but lacks the conceptualization typical of attentive sense perception.
   
2. “States of affairs” includes the impermanence of all reality and that God is the ground of the self.
   
3. By “realities” one refers to beings and abstract objects. These can be God, Brahman, Nirvana, or the Absolute.
   
4. “Acquaintance” of realities means the person or subject having the mystical experience is aware of the presence of these realities. For example, the subject’s awareness of God (a reality) affords an awareness of his utter dependence on God (a state of affairs).
   
5. Mystical experience is noetic, meaning that it involves knowledge of what a subject perceives.

This definition excludes experiences such as religious visions or auditions, out-of-body experiences, telepathy, precognition, and clairvoyance because these involve an acquaintance with objects or qualities of a kind accessible to the senses or to ordinary introspection, such as human thoughts and future physical events.

References

1. Gellman, Jerome. 2004. “Mysticism and Religious Experience.” In The Oxford Handbook of Philosophy of Religion, edited by William J. Wainwright, 138-167. Oxford: Oxford University Press.