Esoteric knowledge is that which is available only to a narrow circle of "enlightened", "initiated", or specially educated people. Esoteric items may be known as esoterica. In contrast, exoteric knowledge is knowledge that is well-known or public; or perceived as informally canonic in society at large.
In Western, English-speaking societies today, the term, "esotericism," is not necessarily used in the sense of mystical knowledge or practice, but has come informally to mean any perception or knowledge that is difficult to understand or remember, such as theoretical physics, or else that which pertains to the minutiae of a particular discipline, such as "esoteric" baseball statistics.
The term esoteric first appeared in English in the 1701 History of Philosophy by Thomas Stanley, in his description of the mystery-school of Pythagoras; the Pythagoreans were divided into "exoteric", (under training}, and "esoteric" (admitted into the "inner" circle).
"Esotericism" sometimes suggests an additional element of initiation. 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 Roman Empire had several mystery religions which emphasized initiation. Some saw Christianity, with its ritual of baptism, as a mystery religion. The terms "Gnosticism" and "Gnosis" refer to a family of religious movements which claimed to possess secret knowledge (gnosis). Another important movement from the ancient world was Hermeticism or Hermetism.
The Ismaili Muslims also stress a distinction between the inner and the outer. It is believed that spiritual salvation is attained by receiving the 'Nur' (light) through the esoteric, that is, spiritual search for enlightenment.
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.
In the 17th century, European esotericism was reformulated 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. Anthroposophy, a synthesis of Western esoteric traditions and Theosophy founded by Rudolf Steiner in the early part of the 20th century, stimulated developments in education, agriculture, and medicine. Yet another notable esoteric strain stems from the teachings of G. I. Gurdjieff and P. D. Ouspensky.