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.
Phonemically, the syllable is /aum/, which is regularly monophthongized to [õː] in Sanskrit phonology. 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 6.1.95 (i.e. ‘om’).
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