from
The Collaborative International Dictionary of English v.0.48
Om \Om\, interj. & n. [Also {Aum}, {Um}.] [Skr. [=o]m.]
A mystic syllable or ejaculation used by Hindus and Buddhists
in religious rites, -- orig. among the Hindus an exclamation
of assent, like Amen, then an invocation, and later a symbol
of the trinity formed by Vishnu, Siva, and Brahma.
{Om mani padme hun}, a sacred formula of buddhism (esp. of
the Lamaists) translated "O, the Jewel in the Lotus,
Amen," and referring to Amitabha, who is commonly
represented as standing or sitting within a lotus.
[Webster 1913 Suppl.]