Mysticism encompasses religious traditions of human transformation aided by various practices and religious experiences. Popularly, mysticism is used synonymously with mystical experience, a neologism which refers to an ecstatic unitive experience of becoming one with God, the Absolute, or all that exists.

Scholarly research since the 1970s had questioned this understanding, noting that what appears to be mysticism may also refer to the attainment of insight into ultimate or hidden truths, as in Buddhist awakening and Hindu prajna, in nondualism, and in the realisation of emptiness and ego-lessness, and also to altered states of consciousness such as samadhi.

The term "mysticism" has Ancient Greek origins with various historically determined meanings. Derived from the Greek word μύω múō, meaning "to close" or "to conceal", mysticism came to refer to the biblical, liturgical (and sacramental), spiritual, and contemplative dimensions of early and medieval Christianity. During the early modern period, the definition of mysticism grew to include a broad range of beliefs and ideologies related to "extraordinary experiences and states of mind".

Broadly defined, mysticism as a way of personal transformation can be found in a number of religious traditions, including Western mysticism and Western esotericism, Sufism, Buddhism, and Hinduism.

"Mysticism" is derived from the Greek μύω, meaning "I conceal", and its derivative μυστικός, mystikos, meaning 'an initiate'. The verb μύω has received a quite different meaning in the Greek language, where it is still in use. The primary meanings it has are "induct" and "initiate". Secondary meanings include "introduce", "make someone aware of something", "train", "familiarize", "give first experience of something".

The related form of the verb μυέω (mueó or myéō) appears in the New Testament. As explained in Strong's Concordance, it properly means shutting the eyes and mouth to experience mystery. Its figurative meaning is to be initiated into the "mystery revelation". The meaning derives from the initiatory rites of the pagan mysteries. Also appearing in the New Testament is the related noun μυστήριον (mustérion or mystḗrion), the root word of the English term "mystery". The term means "anything hidden", a mystery or secret, of which initiation is necessary. In the New Testament it reportedly takes the meaning of the counsels of God, once hidden but now revealed in the Gospel or some fact thereof, the Christian revelation generally, and/or particular truths or details of the Christian revelation.

According to Thayer's Greek Lexicon, the term μυστήριον in classical Greek meant "a hidden thing" or "secret". A particular meaning it took in Classical antiquity was a religious secret or religious secrets, confided only to the initiated and not to be communicated by them to ordinary mortals. In the Septuagint and the New Testament the meaning it took was that of a hidden purpose or counsel, a secret will. It is sometimes used for the hidden wills of humans, but is more often used for the hidden will of God. Elsewhere in the Bible it takes the meaning of the mystic or hidden sense of things. It is used for the secrets behind sayings, names, or behind images seen in visions and dreams. The Vulgate often translates the Greek term to the Latin sacramentum (sacrament).

The related noun μύστης (mustis or mystis, singular) means the initiate, the person initiated to the mysteries. According to Ana Jiménez San Cristobal in her study of Greco-Roman mysteries and Orphism, the singular form μύστης and the plural form μύσται are used in ancient Greek texts to mean the person or persons initiated to religious mysteries. These followers of mystery religions belonged to a select group, where access was only gained through an initiation. She found that the terms were associated with the term βάκχος (Bacchus), which was used for a special class of initiates of the Orphic mysteries. The terms are first found connected in the writings of Heraclitus. Such initiates are identified in texts with the persons who have been purified and have performed certain rites. A passage of Cretans by Euripides seems to explain that the μύστης (initiate) who devotes himself to an ascetic life, renounces sexual activities, and avoids contact with the dead becomes known as βάκχος. Such initiates were believers in the god Dionysus Bacchus who took on the name of their god and sought an identification with their deity.