The Illuminati is the name of many groups, modern and historical, real and fictitious, verified and alleged. Most commonly, however, The Illuminati refers specifically to the Bavarian Illuminati, perhaps the least secret of all secret societies in the world, described below. Most use refers to an alleged shadowy conspiratorial organization which controls world affairs behind the scenes, usually a modern incarnation or continuation of the Bavarian Illuminati. Illuminati is sometimes used synonymously with New World Order.
Illuminati is a Greek word meaning Illumination a name given to those who submitted to Christian baptism. Those who were baptized were called Illuminati or Illuminated / Enlightened Ones by the Ante-Nicene clergy, on the assumption that those who were instructed for baptism in the Apostolic faith had an enlightened understanding. The Alumbrados, a mystical 16th-century Spanish sect, were among the societies that subsequently adopted the name Illuminati.
Since Illuminati literally means 'enlightened ones' in Latin, it is natural that several unrelated historical groups have identified themselves as Illuminati. Often, this was due to claims of possessing gnostic texts or other arcane information not generally available.
The designation illuminati was also in use from the 14th century by the Brethren of the Free Spirit, and in the 15th century was assumed by other enthusiasts who claimed that the illuminating light came, not by being communicated from an authoritative but secret source, but from within, the result of exalted consciousness, or "enlightenment". (Source - Crystallinks)