Om the word Naam can not be defined; here are some definations as understood by some.
Om in Hinduism
vedam pavitram omkara
" What is to be known, that I'm purifier and the syllable OM." - Krishna
Aum or Om (in Devanagari ॐ) is one of the most sacred symbols in Hinduism. In Sanskrit known as praṇava (प्रणव) lit. "to sound out loudly" or oṃkāra (ओंकार) lit. "oṃ syllable")
Hindus consider Aum to be the universal name of the Lord and that it surrounds all of creation. The sound emerging from the vocal cords starts from the base of the throat as "A." With the coming together of the lips, "U" is formed and when the lips are closed, all sounds end in "M."
Om symbolizes the Supreme Personality of Godhead i.e Parabrahman. It symbolizes the prana or life breath which runs through one's body bestowed by the Parabrahman. The significance and explanation of Om is given in the Mandukya Upanishad.
Hindus believe that the essence of the Vedas (ancient Indian scriptures) is enshrined in the word Aum. The belief that the Lord started creating the world after chanting "aum" and "atha" gives this religious symbol a fundamental relevance to the Hindu view of creation. Hence, its sound is considered to create an auspicious beginning for any task that one may undertake.
तस्य वाचकः प्रणवः ॥२७॥ His word is Om. — Yogasutra 1.27
Therefore, uttering Om, the acts of yagna (fire ritual), dāna (charity) and tapas (austerity) as enjoined in the scriptures, are always begun by those who study the Brahman. — Bhagavad Gita 17.24
Ek Onkar/Omkar in Sikhism
This symbol ੴ pronounced Ek Onkar is the symbol that is used to represents the "One Supreme Reality" or "One God."This is the symbol that appears at the beginning of the Guru Granth Sahib. Its the icon that starts the holy text which was first written by Guru Nanak.
This fundamental teaching of Sikhism, is that there is only "one Essence" or "one reality" that sustains all; this is paramount to the understanding of Sikh beliefs.
Bhai Gurdas Ji says of Ek-Onkar:
By writing 1 (One) in the beginning, it has been shown that Ekankar, God, who subsumes all forms in Him is only one. Ura, the first Gurmukhi letter, in the form of Oankar shows the world controlling power of that one Lord...
Om in Buddhism
Om is often used in some later schools of Buddhism, for example Tibetan Buddhism, which was influenced by Indian Hinduism and Tantra. In Chinese Buddhism, Om is often transliterated as the Chinese character 唵 (pinyin ǎn) or 嗡 (pinyin wēng).
Tibetan Buddhism (Vajrayana)
In Tibetan Buddhism, Om is often placed at the beginning of mantras and dharanis. Probably the most well known mantra is "Om mani padme hum", the six syllable mantra of the Bodhisattva of compassion, Avalokiteśvara. This mantra is particularly associated with the four-armed Shadakshari form of Avalokiteśvara. Moreover, as a seed syllable (bija mantra), Aum is considered sacred and holy in Esoteric Buddhism.
Some scholars interpret the first word of the mantra oṃ maṇipadme hūṃ to be auṃ, with a meaning similar to Hinduism – the totality of sound, existence and consciousness.
Oṃ has been described by the 14th Dalai Lama as "composed of three pure letters, A, U, and M. These symbolize the impure body, speech, and mind of everyday unenlightened life of a practitioner; they also symbolize the pure exalted body, speech and mind of an enlightened Buddha." According to Simpkins, Om is a part of many mantras in Tibetan Buddhism and is a symbolism for "wholeness, perfection and the infinite".
In Theravada Buddhism, especially the schools practised in Thailand, there have been a reinterpretation of "Om" or "Aum" to represent the unified Three Jewels, or the Triple Gem, the supreme trinity of Refuge in Buddhism, where the 'A" sound symbolizes the Enlightened Buddha who is an Arahant, the "U" sound represents the ultimate or sublime Dharma, and the "M" sound stands for the multitude of the Sangha. King Mongkut, Rama IV of Siam, who was himself a Pali scholar, authored a Buddhist chant in Pali explaining the Om or Aum sound in term of the Three Jewels, which is still chanted in Thai Buddhist temples.
Meaning and Power of Om
Secret Behind the Aum Chanting By Sadguru