Source : Webster's Revised Unabridged Dictionary (1913)

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.