According to Peter Moore, the term "mysticism" is "problematic but indispensable. It is a generic term which joins together into one concept separate practices and ideas which developed separately, According to Dupré, "mysticism" has been defined in many ways, and Merkur notes that the definition, or meaning, of the term "mysticism" has changed through the ages. Moore further notes that the term "mysticism" has become a popular label for "anything nebulous, esoteric, occult, or supernatural."
Parsons warns that "what might at times seem to be a straightforward phenomenon exhibiting an unambiguous commonality has become, at least within the academic study of religion, opaque and controversial on multiple levels". Because of its Christian overtones, and the lack of similar terms in other cultures, some scholars regard the term "mysticism" to be inadequate as a useful descriptive term. Other scholars regard the term to be an inauthentic fabrication, the "product of post-Enlightenment universalism."
Union with the Divine or Absolute and mystical experience
Deriving from Neo-Platonism and Henosis, mysticism is popularly known as union with God or the Absolute. In the 13th century the term unio mystica came to be used to refer to the "spiritual marriage," the ecstasy, or rapture, that was experienced when prayer was used "to contemplate both God’s omnipresence in the world and God in his essence." In the 19th century, under the influence of Romanticism, this "union" was interpreted as a "religious experience," which provides certainty about God or a transcendental reality.
An influential proponent of this understanding was William James (1842–1910), who stated that "in mystic states we both become one with the Absolute and we become aware of our oneness."[William James popularized this use of the term "religious experience" in his The Varieties of Religious Experience, contributing to the interpretation of mysticism as a distinctive experience, comparable to sensory experiences. Religious experiences belonged to the "personal religion," which he considered to be "more fundamental than either theology or ecclesiasticism". He gave a Perennialist interpretation to religious experience, stating that this kind of experience is ultimately uniform in various traditions.
McGinn notes that the term unio mystica, although it has Christian origins, is primarily a modern expression.McGinn argues that "presence" is more accurate than "union", since not all mystics spoke of union with God, and since many visions and miracles were not necessarily related to union. He also argues that we should speak of "consciousness" of God's presence, rather than of "experience", since mystical activity is not simply about the sensation of God as an external object, but more broadly about "new ways of knowing and loving based on states of awareness in which God becomes present in our inner acts."
However, the idea of "union" does not work in all contexts. For example, in Advaita Vedanta, there is only one reality (Brahman) and therefore nothing other than reality to unite with it—Brahman in each person (atman) has always in fact been identical to Brahman all along. Dan Merkur also notes that union with God or the Absolute is a too limited definition, since there are also traditions which aim not at a sense of unity, but of nothingness, such as Pseudo-Dionysius the Areopagite and Meister Eckhart. According to Merkur, Kabbala and Buddhism also emphasize nothingness. Blakemore and Jennett note that "definitions of mysticism [...] are often imprecise." They further note that this kind of interpretation and definition is a recent development which has become the standard definition and understanding.
According to Gelman, "A unitive experience involves a phenomenological de-emphasis, blurring, or eradication of multiplicity, where the cognitive significance of the experience is deemed to lie precisely in that phenomenological feature".