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.]