Aum (Sk.). The sacred syllable; the triple-lettered unit; hence the trinity in one.
Om or Aum (Sk.). A mystic syllable, the most solemn of all words in India. It is “an invocation, a benediction, an affirmation and a promise”; and it is so sacred, as to be indeed the word at low breath of occult, primitive masonry. No one must be near when the syllable is pronounced for a purpose. This word is usually placed at the beginning of sacred Scriptures, and is prefixed to prayers. It is a compound of three letters a,u,m, which, in the popular belief, are typical of the three Vedas, also of three gods—A (Agni) V (Varuna) and M (Maruts) or Fire, Water and Air. In esoteric philosophy these are the three sacred fires, or the “triple fire” in the Universe and Man, besides many other things. Occultly, this “triple fire” represents the highest Tetraktys also, as it is typified by the Agni named Abhimânin and his transformation into his three sons, Pâvana, Pavamâna and Suchi, “who drinks up water”, i.e., destroys material desires. This monosyllable is called Udgîtta, and is sacred with both Brahmins and Buddhists.
—Theosophical Glossary (HPB)
OM, the name of the Deity, considered as sacred by the Brahmans and Buddhists alike. Its sounds are said by them to contain a mystery and to symbolize the universe. Its full form is Aum. The first sound, in its utterance,—a sound of a—represents Brahma, and signifies creation; its second sound—a sound of u—represents Vishnu, and signifies the preservation of the universe; the third, or “stoppage”—the sound of m—represents Siva, and signifies destruction. Its occult significance is very great. Its substitute word is Pranava.—Working Glossary (WQJ)
Aum (Encyclopedic Theosophical Glossary)
AUM!, article by William Q. Judge
Praṇava (Aum) (Theosophy Wiki)
See also: Mandukya Upanishad