The symbol Om (also called Pranava), is the most sacred symbol in Hinduism. Volumes have been written in Sanskrit illustrating the significance of this mystic symbol. Although this symbol is mentioned in all the Upanishads and in all Hindu scriptures, it is especially elaborated upon in the Taittiriya, Chandogya and Mundaka Upanishads.

The goal, which all Vedas declare, which all austerities aim at, and which human desire when they live a life continence, I will tell you briefly it is Om. The syllable Om is indeed Brahman. This syllable Om is the highest. Whosoever knows this symbol obtains all that he desires. This is the best support; this is the highest support. Whosoever knows this support is adored in the world of Brahman.

— Katha Upanishad I, ii, 15-17

The symbol of Om contains of three curves, one semicircle and a dot. The large lower curve symbolizes the waking state; the upper curve denotes deep sleep (or the unconscious) state, and the lower curve (which lies between deep sleep and the waking state) signifies the dream state. These three states of an individual’s consciousness, and therefore the entire physical phenomenon, are represented by the three curves. The dot signifies the Absolute (fourth or Turiya state of consciousness), which illuminates the other three states. The semicircle symbolizes maya and separates the dot from the other three curves. The semicircle is open on the top, which means that the absolute is infinite and is not affected by maya. Maya only affects the manifested phenomenon. In this way the form of Om symbolizes the infinite Brahman and the entire Universe.

Uttering the monosyllable Om, the eternal world of Brahman, One who departs leaving the body (at death), he attains the superior goal.

— Bhagavad Gita, 8.13