Ik Onkar, or Ik Oankar, (Panjabi, “one”; Sanskrit, “sacred syllable Om“), in Sikhism, Guru Nanak stated that God is one. Ik Onkar is the Sikhs most frequent statement about God, emphasing unity of the Prmal Being in contrast to the three principles (trimurti, Brahma, Vishnu, Shiva) of Hinduism. In its customary symbolic form the numeral stresses the divine singularity or unity. It marks the beginning of the Mul Mantra, and thus of the Japi, and the Adi Gtanth, and prefaces the hymns of each rag--and within these compositions, each Guru and Bhagat. A.G.H.
Bowker, John, The Oxford Dictionary of World Religions, New York, Oxford University Press, 1997, p. 468