What does "om" mean?

Om: 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.