Thelema (Greek: Theta - Epsilon - Lamda - Eta - Mu - Alpha) refers to the system of Magick espoused by Aleister Crowley (1875 - 1947). The word "Thelema" itself is Greek for "Will." The origin of Thelema dates to 1904, when Crowley, whilst residing in Cairo, channelled a text known as The Book of the Law. This envisioned the proclamation of the Aeon of Horus, with Crowley as its prophet.
In practice, Thelema bears many similarities to the magical system of the Golden Dawn of which Crowley was a former member: indeed Crowley published many of the order's private documents in his periodical, "The Equinox". However, Thelema departs from the Golden Dawn in a number of details, which have their basis in the Book of the Law itself and in Crowley's subsequent writings, such as The Holy Books of Thelema and The Vision and The Voice.
There are several Thelemic organisations: two of the most famous are the A.'.A.'., and the OTO. The A.'.A.'. is a highly secretive initiatory organisation which is subject to a lot of unsubstantiated rumour and speculation. The OTO ("Ordo Templi Orientis") is a quasi-Masonic body, which teaches in its highest degrees what is termed "the supreme secret of Freemasonry."