Such definitions inevitably emphasize the negative aspects of the experience (because it is only negative aspects that are shared by many religions and no religion) - and that mystical experience is beyond normal so is ineffable - cannot (or should not) be described in words.
But that is not distinctive to mystical experience - because many, many experiences cannot be described of themselves - music, smells and tastes, love...
Another descriptor is that mystical experience includes a cessation of thinking, loss of the self - hence absorption into reality, into 'the universe'.
But negation of what is distinctive about Man, a cessation-of Man's higher faculties, can equally be conceptualized as a devolution towards the animal...
Loss of consciousness; loss of functioning (e.g. due to disease or extreme intoxication) and the like are achievable by deep sleep, and death - at least as death is conceptualized by modernity.
Such a mysticism tends towards nihilism and desired-destruction.
Other - more positive - descriptions of the mystical, focus on perceptual experiences.
For example seeing visions; or hearing voices providing inspired information, or prophecy, or answering questions.
A person in a trance state who 'channels' what purports to be the voice of some higher being, angel or god is also in this 'perceptual' category - albeit the channeler may not be aware of what is being-said, while it is being said.
The sense of touch may be the route for physical feelings including emotional changes, or a sense of being-controlled. Another touch-related idea is bliss - that mystical experience includes a trance-like blissful state which can be interpreted as a bodily (i.e. sense of inner sensation) experience; and indeed, mystical bliss can apparently be replicated, or closely simulated, by physical interventions includes some drugs.
And this being physically-inducible, of course, tends to invalidate the spiritual significance of bliss
Indeed, the spiritual significance of all such perceptual mystical experiences need to be interpreted and understood - just as any other communications in mundane life need to be understood. The meaning of a vision or a prophecy may therefore be unclear, or become contentious.
There is always the possibility of explaining-away and mystical perception as caused by some kind of pathological state - merely an hallucination.
Furthermore, perceptual physical experiences include a desire for passivity; a desire to be overwhelmed - and the implicit notion that validity comes from this passively being-overwhelmed.
This, in turn, links perceptual mysticism to desired submission to power - including earthly power; especially when earthly power can provide (or claims to provide) access t perceptual mystical experiences by immersion in media. Such transhumanistic, techno-mysticism has been a subcultural idea for several decades.
In the end, for Modern Man, these universalist descriptors of mysticism do not differentiate genuine spiritual experience from mundane experiences.
This has several important implications. One is that mysticism is, for most people and all of public discourse - thoroughly discredited. General culture has, thus, provided multiple conceptual tools for tearing-down and dismissing any mystical experience which anybody tries to communicate to others.
This means that henceforth mystical experience is - in effect - a private and subjective phenomenon; because the attempt to make it public and objective will almost certainly fail.
The problem of interpreting and understanding perceptual mysticism is one that will not go away - even someone who experiences a vision, or hears a voice; may be unsure of what it really means... unless that perception was accompanied by a 'direct' (wordless) conviction of knowing, and of the validity of that knowing.
This means that here-and-now; it is not the 'spectacular' aspects of mysticism that are most important - the visions, speaking from heaven, states of bliss, trances and channeled voices or the like. What is informationally most important is that (private, subjective, not-perceptual) strong and inward sense of knowing - accompanied by conviction that such knowledge is valid.
But, as with the mystics of old, this is not a merely intellectual phenomenon, but is instead a whole-person phenomenon - one that includes the body, and therefore the emotions.
Part of the 'validation', the sense of conviction, of knowing truth (constrained only by our capacity for truth; and our need and desire for truth) - is that it is part of a different and higher (i.e. more divine, more in harmony with God) conscious state - a situation that is deeply satisfying to experience. In other words; the mysticism I describe is a complete conscious situation - including both knowledge experienced without mediation, and the validation of that knowledge by conviction that is part of a distinctive emotional state.
My overall conclusion is that mysticism cannot usefully be defined in a way that makes it apply to all religions or no religion - and such attempts fail to distinguish real mysticism from various other mundane states.
We therefore need to accept that 'modern mysticism' is primarily for the benefit of each individual who experiences it as a direct contact with the divine; and that individual should not expect to be able to convince others of either the validity or content of his experience.
The days of mystics having "messages for humanity" are over. And, indeed, most such mystics from the 19th and 20th centuries (even those who were not merely self-deluded or charlatans) comprehensively discredited themselves by generating too much, too specific and detailed public information; especially cosmologies and prophecies.
On the plus side, by my understanding of what might be termed 'modern mysticism' - it is a far more common phenomenon and experience than was the old style.
It may even be universal among Christians - it may be that all Christians have mystical experiences, and quite frequently...
However; these experiences are seldom being-recognized as mystical; since people are looking for something less conscious and more 'spectacular' (and suitable for 'public consumption') than the nature of simple, direct knowing - a knowing tailored (divinely) for the specific current needs of the recipient... And sustained by a euphoria that is not like overwhelming bliss; but more like the controlled, partly-self-generated, deep-satisfaction experienced during active creating.
Thus mysticism is related to the divine providence which rules each and every life; mysticism is an important way in which we discover or confirm our providence.
In a nutshell, I would advocate a metaphysics of mysticism that regards the experience as a type of creativity, and indeed of creative genius; with the understanding that this is the genius of our-selves and for our-selves - that is, our true and divine selves*.
And I would advocate that people become aware of this experience as a possibility, and notice it as an actuality.
It will then - I think - be found that (almost) every Christian is a mystic.
*Which was indeed something like the original meaning of genius.