Just Say Om!
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There is so much to explain concerning the word Om and the symbol that I thought it best to have this link to Wikipedia which explains it much better than I could.
Click here for the Wikipedia, the free encyclopedia article
Aum (also Om, ) is the most sacred syllable in Hinduism, symbolizing the infinite Brahman and the entire Universe. This syllable is sometimes called the "Udgitha" or "pranava mantra" (primordial mantra), because it is considered to be the primal sound, and because most mantras begin with it. It first came to light in the Vedic Tradition. As a seed syllable (bija), it is also considered holy in Esoteric Buddhism.