Illuminati is a term that has been used by occultists since the late 15th century onwards to described spiritual adepts who received mystical insights or «illuminations» from a transcendent source.
The Order of the Illuminati was founded by a Bavarian professor of law Adam Weishaupt in 1776, but it was hardly esoteric in any mystical sense because most of its «secrets» were based on the works of Voltaire and other French Encyclopedists.
Weishaupt and another enthusiast Adolph Knigge later adapted the Order’s teachings in order to infiltrate Freemasonry. A decree in Bavaria in 1784 banned all secret societies – including Freemasonry – and the Order decline. However, by the turn of the 20th century occultists Leopold Engel and Theodor Reuss revived it. In recent times the fantasy occult writer Robert Anton Wilson has popularized the idea of a secret brotherhood of adepts. A.G.H.
Sources: 62, 126.