Of the many sacred syllables in Hinduism which resonate with different properties of the cosmos, OM is the most scared.  It is considered to be the primordial sound of the cosmos itself, intoned when the universe was created. OM sonically represents Brahman, the ineffable Supreme Reality, and thus naturally sanctifying. For this reason it is intoned at the beginning of every mantra or prayer. It is also considered a condensed version of all scared writings. It is sometimes written AUM and pronounced "ah-oo-mm" but the meaning is the same.