Let a man meditate on the syllable Om, called the udgîtha; for the udgîtha (a portion of the Sâma-veda) is sung, beginning with Om.

The full account, however, of Om is this:–

The essence of all beings is the earth, the essence of the earth is water, the essence of water the plants, the essence of plants man, the essence of man speech, the essence of speech the Rig-veda, the essence of the Rig-veda the Sâma-veda, the essence of the Sâma-veda the udgîtha (which is Om).

That udgîtha (Om) is the best of all essences, the highest, deserving the highest place, the eighth.

What then is the Rik? What is the Sâman? What is the udgîtha? ‘This is the question.

KHÂNDOGYA-UPANISHAD