Mysticism is the belief that knowledge of God or Ultimate Reality can (only) be gained through personal experience.
Individual experience of God
In theistic, pantheistic, and panentheistic classical pantheist/cosmotheist metaphysical systems this is most often understood as individual experience with God or in some cases, Goddess. Thomas Aquinas defined it simply as "cognitio dei experimentalis." The experiences are very subjective, and they are related to others as visions, dreams, revelations, prophecies, and so forth.
Introspective experience of Reality
In philosophy, the term Perennial Philosophy is used, and relates to a primary concern "with 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 assistance of drugs.
Mysticism is unifying in nature
Mystics of different traditions show that they have known similar experiences of a world usually outside the five normal senses. Therefore it is argued that the true unity of religion and philosophy can be found in mystical experience.
Elements of mysticism are present in most religions and many philosophies. Some mystics claim that there is a common thread of influence in all mystic philosophies that is traceable back to a shared source. The Vedic tradition is inherently mystic; the Christian apocalyptic Book of Revelation is clearly mystic, as with Ezekiel's or Daniel's visions of Judaism, and the Koran was inspired by the angel Gabriel in a mystic manner. Indigenous cultures also have cryptic revelations pointing toward a universal flow of love or unity, usually following a vision quest, or similar rituals. Mystical philosophies thus exhibit a strong tendency towards syncretism.
On the difficulty of defining mysticism
Gershom Scholem was the most prominent scholar of the twentieth century dedicated to the academic study of Kabbalah. In his work, Kabbalah, he stated:
The Kabbalah is not a single system with basic principles which can be explained in a simple and straightforward fashion, but consists rather of a multiplicity of different approaches, widely separated from one another and sometimes completely contradictory.
These kinds of open-ended statements are found throughout the history of mysticism.
Mysticism is growing in popularity
Across the spectrum, the 19th Century saw an increase of interest in mysticism linked to an interest in Occultism and Eastern thought. Major figures in this movement, called Theosophy, were Madame Blavatsky and Gurdjieff. This movement has had a later influence on the New Age. Raymond Moody documented 10,000 cases of Near Death Experiences which are mystical in nature, as the individual typically sees God or heaven. Books like Conversations with God hit the bestsellers lists, rather than resting in relative obscurity for 300 years.
Examples in major traditions
Examples of major traditions and philosophies with strong elements of mysticism are Vedantic Hinduism, Tibetan and Zen Buddhism, the Christian Gnostic sects, Mystery Religions and Cults, the Eastern Orthodox hesychasm, the Sufi school of Islam, the Judaic Kabbalah and many aspects of the New Age movement, such as Near Death Experiences. The list is endless: Quakerism has a strong mystical element to its theology, and like Mohammed, the American prophet Joseph Smith, Jr. began a religion on the strength of visions, revelations, and ordinations from angels, and the Ghost Dances of the late 19th Century were mystical in origin.
Some examples of Hindu mystics:
Some examples of Christian mystics:
Some examples of Muslim mystics (also called sufi):
Some examples of Jewish mystics:
Some examples of other mystics: