OM

In Vedanta, OM (pronounced, and sometimes spelled as, AUM) is the eternal, mystical syllable.  It is the mantra of Brahman.

Swami Vivekananda described the mantra this way:  "The first letter, A, is the root sound, the key, pronounced without touching any part of the tongue or palate; M represents the last sound in the series, being produced by the closed lips; and the U ( in the middle) rolls from the very root to the end of the sounding-board of the mouth.  Thus, OM represents the whole phenomenon of sound producing."  All sounds are contained within it.

The sound A-U-M comprises a triangle, the symbol of three in one.  Specifically, it is the three ages contained in the Soul, that is, time past, time present, and the eternal future.  It is the three divine powers:  creation, preservation, and transformation.  And it is the three essences:  immortality, omniscience, and joy in Brahman.

Repetition of OM while meditating on its meaning is a powerful spiritual practice in Vedanta.

In Tibetan Buddhism, OM represents pure spirit, and is found in the mantra OM man-ni pad-me hum, which translates as "OM!  The jewel in the lotus!  Hum!" 

OM man-ni pad-me hum has manifold esoteric interpretations having to do with liberation.  For instance, each syllable of the mantra has a specific significance:  OM closes the door to rebirth among the gods; MA, to rebirth among demons; NI, among mankind; PAD, among subhuman creatures; ME, among unhappy ghosts; HUM, among inhabitants of hell.