A wide range of perspectives occur among spiritual traditions and beliefs which embrace direct experiential knowledge of God, Divinity, or Ultimate Reality. Different traditions adopt a range of intellectual or rational assessments of what is likely, possible, provable, approvable or factual. Among these the idea of union or interrelationiship of oneself and of all mortal beings with the ultimate imperishable being is often declared to be something that can be experienced in profound, definite and personally undeniable ways, rather than something that is merely conjectured. It is often asserted that the triggering of such experience can involve ritual prayer and contemplations focused on such union, or may sometimes occur spontaneously with some individuals.
In theistic, pantheistic, and panentheistic classical pantheist/cosmotheist metaphysical systems, mystical experience is most often understood as individual communion with a god or goddess. These experiences are very subjective, and they may be experienced as visions, dreams, revelations, prophecies, and so forth.
StThomas Aquinas, a Catholic mystic from the 13th century, defined it as cognitio dei experimentalis. In Catholicism the mystical experience is not sought for its own sake, and is always informed by revelation and ascetical theology. This causes the subjectivist tendency of mysticism to be curtailed, as experiences not aligned with truths otherwise known are discarded.
"[W]ith the one, divine reality substantial to the manifold world of things and lives and minds. But the nature of this one reality is such that it cannot be directly or immediately apprehended except by those who have chosen to fulfill certain conditions, making themselves loving, pure in heart, and poor in spirit." — Aldous Huxley
Some mystics use the term to refer to a manner wherein the mystic plumbs the depths of the self and reality in a radical process of meditative self-discovery to discover the true nature of reality experientially. This can happen with or without the use of mind-altering substances.
Some systems of mysticism are found within specific religious traditions and do not relinquish doctrinal principles as a part of mystical experience. For example, Christian mystics, through the centuries, have not decided that Jesus is not God after all: in other words, not all mysticism results in syncretism. In some definite cases, theology remains a distinct source of insight that guides and informs the mystical experience. For example, StThomas Aquinas' mystical experiences all occurred squarely within the love of the Catholic Eucharist.
In Catholic traditions, mystical theology is informed by revelation, which averts an apparent tendency to become lost in formless thought. Christian mystics, too, are obligated to obey the forms of ascetical and moral theology, as following Christ is their primary objective, rather than seeking mystical experiences for their own sake. 
The late 19th century saw an significant increase of interest in mysticism in the West that was combined with increased interest in Occultism and Eastern Philosophy. Theosophy was a major movement in the popularization of these interests. Madame Blavatsky and Gurdjieff were central figures of the theosophy movement. This trend was later to become absorbed in the rise of the New Age movement. At the end of the 20th Century books like Conversations with God (a series which describes the author's experience of direct communication with God) hit the bestseller lists, and films like The Matrix reached an audience not unfamiliar with its philosophical themes.