Om is also written ओ३म् (ō̄m [õːːm]), where ३ is pluta ("three times as long"), indicating a length of three morae (that is, the time it takes to say three syllables)—an overlong nasalised close-mid back rounded vowel—though there are other enunciations adhered to in received traditions. It is placed at the beginning of most Hindu texts as a sacred incantation to be intoned at the beginning and end of a reading of the Vedas or prior to any prayer or mantra. It is used at the end of the invocation to the god being sacrificed to (anuvakya) as an invitation to and for the latter to partake of.Name, phonology and written representation[edit]
The Sanskrit name for the syllable is praṇava, from a root nu "to shout, sound", verbal pra-nu- being attested as "to make a humming or droning sound" in the Brahmanas, and taking the specific meaning of "to utter the syllable om" in the Chāndogya Upanishad and the Shrauta Sutras. More rarely used terms are akṣara (lit. symbol, character) or ekākṣara (lit. one symbol, character), and in later times omkāra becomes prevalent.
Phonologically, the syllable is /aum/, which is regularly monophthongised to [õː] in Sanskrit. It is sometimes also written with pluti, as o3m (ओ३म्), notably by Arya Samaj. When occurring within a Sanskrit utterance, the syllable is subject to the normal rules of sandhi in Sanskrit grammar, however with the additional peculiarity that after preceding a or ā, the au of aum does not form vriddhi (au) but guna (o) per Pāṇini