Aum is the principle mantra in Hindu and Tantric Buddha practice, also translated as OM in some sources. It has been adopted into many Western occult systems since the rise of Theosophy in the late 19th century. In Hindu writings it is considered the origin of the alphabet and of speech. A.G.H.
Greer, John Michael. The New Encyclopedia of the Occult. St. Paul. MN. Llewellyn Worldwide. 2005. p. 51