Surely one of the most useful definitions of Western esotericism I have ever come across (also Western occult tradition, Western hermetic tradition, Western mystery tradition) is that of Antoine Faivre, the first to define Western esotericism as a field of interdisciplinary academic study.
In his book Western Esotericism, Faivre listed six characteristics, four which he found fundamental, two which he found often in close association. The four fundamental characteristics are as follows:
1. The idea of universal correspondences. Non-“causal” correspondences operate between all the levels of reality of the universe, which is a sort of theater of mirrors inhabited and animated by invisible forces. For example, there would exist relationships between the heavens (macrocosm) and the human being (microcosm), between the planets and the parts of the human body, between the revealed texts of religions (the Bible, principally) and what Nature shows us, between these texts and the History of humanity.
2. The idea of living Nature. The cosmos is not only a series of correspondences. Permeated with invisible but active forces, the whole of Nature, considered as a living organism, as a person, has a history, connected with that of the human being and of the divine world. To that are often added interpretations, heavy with implications, of the passage from Romans 7:19–22 according to which suffering Nature, subject to the exile and to vanity, also awaits its deliverance.
3. The role of mediations and of the imagination. These two notions are mutually complementary. Rituals, symbols charged with multiple meanings (mandala, Tarot cards, biblical verse, etc.), and intermediary spirits (hence, angels) appear as so many mediations. These have the capacity to provide passages between different levels of reality, when the “active” imagination (the “creative” or “magical” imagination—a specific, but generally dormant faculty of the human mind), exercised on these mediations, makes them a tool of knowledge (gnosis), indeed, of “magical action on the real.”
4. The experience of transmutation. This characteristic comes to complete the three preceding ones by conferring an “experiential” character on them. It is the transformation of oneself, which can be a “second birth”; and as a corollary that of a part of Nature (e.g., in a number of alchemical texts).
As far as the two so-called secondary characteristics are concerned, they are, on the one hand, a practice of concordance:It is a matter of positing a priori that common denominators can exist among several different traditions, indeed among all of them, and then of undertaking to compare them with a view to finding a higher truth that overhangs them. And it is, on the other hand, the emphasis put on the idea of transmission: Widespread in these esoteric currents especially since the eighteenth century, it consists in insisting on the importance of “channels of transmission”; for example, “transmission” from master to disciple, from the initiator to the “initiable” (self-initiation is not possible). To be valuable or valid, this transmission is often considered necessarily to belong to an affiliation whose authenticity (“regularity”) is considered genuine. This aspect concerns the Western esoteric currents especially starting at the time when they began to give birth to initiatic societies (i.e., starting from the mid-eighteenth century).
In practice I have found this definition a useful aid to understanding various occult traditions (including the Hermetic Order of the Golden Dawn, Theosophy, Thelema, Wicca, etc) and symbolic correspondence systems (including Astrology, Alchemy, Astrology, Qabalah, Tarot, etc). Conversely, although this should go without saying, it is also helpful for identifying where Western religious movements are not properly identified as belonging to the Western occult tradition (for instance Christian mysticism, spiritualist churches, the paranormal research community, etc). It is also helpful for understanding those who, although not occultists per se, are nevertheless culturally endebted to occulture.