The word occult comes from the Latin word occultus (clandestine, hidden, secret), referring to "knowledge of the hidden". In the medical sense it is used to refer to a structure or process that is hidden, e.g. an "occult bleed" may be one detected indirectly by the presence of otherwise unexplained anaemia.
The word has many uses in the English language, popularly meaning "knowledge of the paranormal", as opposed to "knowledge of the measurable", usually referred to as science. The term is sometimes popularly taken to mean "knowledge meant only for certain people" or "knowledge that must be kept hidden", but for most practicing occultists it is simply the study of a deeper spiritual reality that extends beyond pure reason and the physical sciences. The terms esoteric and arcane can have a very similar meaning, and the three terms are often interchangeable.
The term occult is also used as a label given to a number of magical organizations or orders, and the teachings and practices as taught by them. The name also extends to a large body of literature and spiritual philosophy.
Occultism is the study of occult or hidden wisdom. To the occultist it is the study of "Truth", a deeper truth that exists beneath the surface: 'The truth is always hidden in plain sight'. It can involve such subjects as magic (alternatively spelled and defined as magick), alchemy, extra-sensory perception, astrology, spiritualism and numerology. There is often a strong religious element to these studies and beliefs, and many occultists profess adherence to religions such as Gnosticism, Hermeticism, Luciferianism, Thelema, and Neopaganism. While Christianity, Judaism, Hinduism, Buddhism, and Islam are generally not considered occult, some of their modern interpretations can be, as the interpretation of Hinduism within Theosophy or the various occult interpretations of the Jewish Kabbalah. Orthodox members of such religions are likely to consider such interpretations false; For example, the Kabbalah Centre has been criticised by Jewish scholars.
The word "occult" is somewhat generic, in that almost everything that isn't claimed by any of the major religions can be considered the occult. Even religious scientists have difficulties in defining occultism. A broad definition is offered by Nicholas Goodrick-Clarke:
"OCCULTISM has its basis in a religious way of thinking, the roots of which stretch back into antiquity and which may be described as the Western esoteric tradition. Its principal ingredients have been identified as Gnosticism, the Hermetic treatises on alchemy and magic, Neo-Platonism, and the Kabbalah, all originating in the eastern Mediterranean area during the first few centuries AD."
From the 15th to 17th century, these kinds of ideas that are alternatively described as Western esotericism had a brief revival. Alchemy used to be common among highly important seventeenth-century scientists, such asIsaac Newton and Gottfried Leibniz. Isaac Newton was accused of introducing occult agencies into natural science when he postulated gravity as a force capable of acting over vast distances. This revival of alchemy and other occult studies was halted by the triumph of empirical sciences and the Age of Enlightenment. "By the eighteenth century these unorthodox religious and philosophical concerns were well defined as 'occult', inasmuch as they lay on the outermost fringe of accepted forms of knowledge and discourse," and were only preserved by a few antiquarians and mystics. However, from about 1770 onwards, a renewed desire for mystery, an interest in the Middle Ages and a romantic temper encouraged a revival of occultism in Europe, "a reaction to the rationalist Enlightenment."
Based on his research into the modern German occult revival 1890-1910, Goodrick-Clarke puts forward a thesis on the driving force behind occultism. Behind its many varied forms apparently lies a uniform function, "a strong desire to reconcile the findings of modern natural science with a religious view that could restore man to a position of centrality and dignity in the universe.
That the Kabbalah has been considered an occult study is also perhaps because of its popularity among magi (the biblical wise men who visited the Infant Jesus are said to have been magi of Zoroastrianism) and Thelemites. Kabbalah was later adopted by the Golden Dawn and brought out into the open by Aleister Crowley and his protégé Israel Regardie. Since that time many authors have emphasized a syncretic approach by drawing parallels between different disciplines.
Direct insight into or perception of the occult does not usually consist of access to physically measurable facts, but is arrived at through the mind or the spirit. The term can refer to mental, psychological or spiritual training. It is important to note, however, that many occultists will also study science (perceiving science as an adjunct to Alchemy) to add validity to occult knowledge in a day and age where the mystical can easily be undermined as flights-of-fancy. An oft-cited means of gaining insight into the occult is the use of a focus; a physical object, a ritualistic action (for example, meditation or chanting), or a medium in which one becomes wholly immersed. These are just a few examples of the vast and numerous avenues that can be explored.
Science and the occult
Occultism is conceived of as the study of the inner nature of things, as opposed to the outer characteristics that are studied by science. The German Kantian philosopher Arthur Schopenhauer designates this 'inner nature' with the term 'Will', and suggests that science and mathematics are unable to penetrate beyond the relationship between one thing and another in order to explain the 'inner nature' of the thing itself, independent of any external causal relationships with other 'things'. Schopenhauer also points towards this inherently relativistic nature of mathematics and conventional science in his formulation of the World as Will. By defining a thing solely in terms of its external relationships or effects we only find its external, or explicit nature. Occultism, on the other hand, is concerned with the nature of the 'thing-in-itself'. This is often accomplished through direct perceptual awareness, known as mysticism.
In the Middle Ages, occult qualities were physical properties that had no rational explanation. At that time magnetism was sometimes called an occult quality. 
Academia & Occultism
There have been various attempts to integrate occultism into an academic framework including a master's program at the University of Amsterdam.
Religion and the occult
Some religious denominations view the occult as being anything supernatural or paranormal which is not achieved by or through God (as defined by those religious denominations), and is therefore the work of an opposing and malevolent entity. The word has negative connotations for many people, and while certain practices considered by some to be "occult" are also found within mainstream religions, in this context the term "occult" is rarely used and is sometimes substituted with "esoteric".
In Judaism, special spiritual studies such as Kabbalah have been allowed for certain individuals (such as rabbis and their chosen students). Furthermore, there are branches of Esoteric Christianity that practice divination, blessings, or appealing to angels for certain intervention, which they view as perfectly righteous, often supportable by gospel (for instance, claiming that the old commandment against divination was superseded by Christ's birth, and noting that the Magi used astrology to locate Bethlehem). Rosicrucianism, one of the most celebrated of Christianity's mystical offshoots, has lent aspects of its philosophy to most Christian-based occultism since the 17th century. Tantra, originating in India, includes amongst its various branches a variety of ritualistic practices ranging from visualisation exercises and the chanting of mantras to elaborate rituals involving sex or animal sacrifice, sometimes performed in forbidden places such as cremation grounds. Tantric texts were at one stage unavailable for mass public consumption due to the social stigma attached to the practices. In general, tantra was predominantly associated with black magic and the tantriks were held in great dishonor.
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- Journal of Thelemic Studies - the first non-partisan, academic journal investigating the occult tradition of Thelema, founded by the infamous Aleister Crowley
- University of Amsterdam Center for Study of Western Esotericism
- University of Exeter Centre for the Study of Esotericism (EXESESO)
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- the MYSTICA.ORG An on-line encyclopedia of the occult