Esotericism refers to the doctrines or practices of esoteric knowledge, or otherwise the quality or state of being described as esoteric, or obscure. Esoteric knowledge is that which is specialized or advanced in nature, available only to a narrow circle of "enlightened", "initiated", or highly educated people. . Items pertaining to esotericism may be known as esoterica.  Some interpretations of esotericism are very broad and include even unconventional and non-scientific belief systems, typically as contrasted with the "scientific" or "traditional religious" beliefs of the society without or "at large". In contrast, exoteric knowledge is knowledge that is well-known or public; or perceived as informally canonic in society at large.
Esoteric is an adjective originating in Greece; it comes from the Greek ἐσωτερικός esôterikos, from esôtero, the comparative form of ἔσω esô: "within". Esoteric refers to anything that is inner. Its antonym is exoteric, from the Greek ἐξωτερικός}} eksôterikos, from eksôtero, the comparative form of ἔξω}} eksô: "outside". Plato, in his dialogue Alcibíades (circa 390 BC), uses the expression ta esô meaning «the inner things», and in his dialogue Theaetetus (circa 360 BC) he uses ta eksô meaning «the outside things». The probable first appearance of the Greek adjective esôterikos is in Lucian of Samosata's "The Auction of Lives", § 26 (also called "The Auction of the Philosophical Schools"), written around AD 166. 
The term esoteric first appeared in English in the 1701 History of Philosophy by Thomas Stanley, in his description of the "Auditors of Pythagoras." The Pythagoreans were divided into "exoteric", which were under review, and "esoteric", which had performed well enough to be admitted into the "inner" circle.
"Esotericism" in current usage
In Western, English-speaking societies today, the term "esotericism" has come to informally mean any knowledge that is difficult to understand or remember, such as theoretical physics, or that which pertains to the minutiae of a particular discipline, such as "esoteric" baseball statistics.
The term "esoteric" does not necessarily refer to "esotericism" per se in the sense of "inner" knowledge, disciplines, or practices.
A variety of past traditions could be classified as forms of "esotericism" due to their "inner" focus as well as their "selective" and "secretive" nature.
Esotericism largely overlaps with "hidden knowledge." Some overlap exists as well between esotericism and mysticism. However, many mystical traditions do not attempt to introduce additional spiritual knowledge, but rather seek to focus the believer's attention or prayers more strongly upon the object of devotion. A mystic is thus not necessarily an esotericist.
"Esotericism" sometimes suggests an additional element of initiation, for example the requirement that one be tested before learning the higher truth. Note however that most "esoteric" teachings are widely available, and indeed often actively promoted.
Another possibility is that such knowledge may be kept secret not by the intention of its protectors, but by its very nature—for example, if it is accessible only to those with the proper intellectual background.
The religiously minded have sometimes used "esotericism" to refer to their belief-systems. For this reason a brief survey of some religious traditions follows. This is not necessarily the meaning of esotericism. Academic esotericism constitutes the modern academic disciplines - looked at in articles under the relevant headings.
Historical sketch of religious ideas
Esotericism is not a single tradition but a vast array of often unrelated figures and movements. Nevertheless, the following may be helpful.
The Roman Empire gave birth not only to Christianity but also to a group of mystery religions which emphasized initiation. Some see Christianity, with its ritual of baptism, as a mystery religion.
Acts 2:38 Peter replied, "Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit.
After Christianity became the state religion of Rome, dissident Christian groups became persecuted as traitors to the state. Pagan groups came to be suppressed as well. The terms "Gnosticism" and "Gnosis" have been challenged as coherent categories, but refer to a family of ancient Jewish, Christian, and pagan religious movements which often did claim to possess secret teachings relating to the spirit world, as opposed to the ordinary world which they tended to denigrate. Another important movement from the ancient world was Hermeticism, sometimes called Hermetism to distinguish it from post-Renaissance appropriations of it. Separately, ancient Babylon provided the basis for Western astrology.
During the Middle Ages such things as astrology, alchemy, and magic were not distinct from the standard subjects of the curriculum of an educated man. While some people assume esotericism to be opposed to the Bible or Christianity, as a historical matter this tension did not arise until later. Indeed, Christianity contributed its own esoteric imagery, notably the Holy Grail from Arthurian literature.
While many esoteric subjects have a history reaching back thousands of years, these have generally not survived as continuous traditions. Rather, they have benefited from various antiquarian revival movements. During the Italian Renaissance, for example, translators such as Ficino and Pico della Mirandola turned their attention to the classical literature of neo-Platonism, and what was thought to be the pre-Mosaic tradition of Hermeticism.
European esotericism was reformulated in the 17th century as Rosicrucianism, and later entered various strands of Freemasonry. In the 19th century a notable French revival in turn gave way to the theosophy of H. P. Blavatsky. In the 20th century Theosophy was reformulated by Annie Besant, C. W. Leadbeater, Alice Bailey, Rudolf Steiner and many others. Theosophy is also considered a major influence on the many current varieties of esotericism in metaphysical organizations, "Ascended Master Activities", and within the New Age groups.
Rudolf Steiner, who broke with theosophy to found his own anthroposophy, spoke of a disagreement between esotericists at the close of the 19th century; one branch wanted to open up esoteric knowledge to the general public, while another group wished to maintain secrecy.  Rudolf Steiner, The Occult Movement in the 19th Century, GA 254, page 139.] Steiner himself claimed to stand in the lineage of those who wanted to make the esoteric an accepted part of mainstream culture. His first books, written in the 19th century, avoided any reference to esoteric themes, but he saw the 20th century as the dawn of a new age, when spirituality would be increasingly central to human development. Thus, he began to publish works such as 'Theosophy' and 'Occult Science' and to lecture on esoteric themes both to select audiences (members of the Anthroposophical Society or of his own esoteric school) and to the general public. All but the most esoteric of these lectures were already being published during his lifetime, and in the last decades even the most esoteric material has been made available by the Rudolf Steiner Archive and Press, Rudolf Steiner Archive and Press in accordance with Steiner's wishes.
- Benjamin Walker, Encyclopedia of Esoteric Man: The Hidden Side of the Human Entity, Routledge and Kegan Paul, London, 1977, ISBN 0-7100-8479-X
- Benjamin Walker, Man and the Beasts Within: The Encyclopedia of the Occult, the Esoteric, and the Supernatural, Stein & Day, New York, 1978, ISBN 0-8128-1900-4
- Wouter J. Hanegraaff (ed.) in collaboration with Antoine Faivre, Roelof van den Broek & Jean-Pierre Brach, Dictionary of Gnosis and Western Esotericism, 2 vols., Brill, Leiden 2005.
- Aries: Journal for the Study of Western Esotericism, Brill, Leiden, since 2001.
- Aries Book Series: Texts and Studies in Western Esotericism, Brill, Leiden, since 2006.
- Antoine Faivre, Access to Western Esotericism, SUNY Press, Albany 1994.
- Antoine Faivre, Theosophy, Imagination, Tradition: Studies in Western Esotericism, SUNY Press, Albany 2000.
- Kocku von Stuckrad, Western Esotericism: A Brief History of Secret Knowledge, Equinox, London / Oakville 2005.
- Wouter J. Hanegraaff, 'The Study of Western Esotericism: New Approaches to Christian and Secular Culture', in: Peter Antes, Armin W. Geertz & Randi R. Warne, New Approaches to the Study of Religion, vol. I: Regional, Critical, and Historical Approaches, Walter de Gruyter, Berlin / New York 2004.
- University of Amsterdam Center for Study of Western Esotericism Research & BA/MA programs in Western esotericism.
- University of Exeter Centre for the Study of Esotericism (EXESESO)
- ESSWE European Society for the Study of Western Esotericism, with many links to associated organizations, libraries, scholars etc.
- Gnosticweb Free courses on esoteric topics such as astral projection and the esoteric path
- Revista Bajo los Hielos René Guénon, Abellio, Evola, Lubicz, etc.
- Libreria Iniciática - Especializada en temática esotérica.
- Timeline of Esoteric History
- Masonic Esotericism
- American Gnostic Association Understanding Esotericism as the root of all religions and systems of spiritual cultivation
- The first Esoteric Center in Lebanon and the Arab World Teaching the esoteric techniques of self-knowledge...