Mantras in both Hindusim and Buddhism contain the syllable OM. The best meditators and yogis in the world are using this syllable to reach enlightenment, so what makes it so powerful? What is the historical significance of the word. How was it discovered and what does it mean?
To explore the meaning of this sacred syllable, lets go to the source. Below is an excerpt from The Yoga Sutras of
Patanjali written in 400CE - the original yogic manuscript. Here we can see a definition of the syllable OM by the creator of yoga himself.
The goal can also be attained via submission to the concept of an ideal being (ishvara).
क्लेश कर्म विपाकाअशयैःअपरामृष्टः पुरुषविशेष ईश्वरः ॥२४॥
kleśa karma vipāka-āśayaiḥ-aparāmr̥ṣṭaḥ puruṣa-viśeṣa īśvaraḥ ॥24॥
Ishavara is a special being that is unaffected by the obstacles of the spiritual aspirant (klesha), specific actions and consequences (karma), or recollections or desires. ||24||
तत्र निरतिशयं सर्वज्ञबीजम् ॥२५॥
tatra niratiśayaṁ sarvajña-bījam ॥25॥
Ishavara is unmatched and is the source of all knowledge. ||25||
स एष पूर्वेषामपिगुरुः कालेनानवच्छेदात् ॥२६॥
sa eṣa pūrveṣām-api-guruḥ kālena-anavacchedāt ॥26॥
Ishvara is each and every one, and is even the teacher of the first ones; he is unaffected by time ||26||
तस्य वाचकः प्रणवः ॥२७॥
tasya vācakaḥ praṇavaḥ ॥27॥
OM is a symbol for ishvara. ||27||
तज्जपः तदर्थभावनम् ॥२८॥
taj-japaḥ tad-artha-bhāvanam ॥28॥
Repetition of OM (with this meaning) leads to contemplation. ||28||
ततः प्रत्यक्चेतनाधिगमोऽप्यन्तरायाभवश्च ॥२९॥
tataḥ pratyak-cetana-adhigamo-'py-antarāya-abhavaś-ca ॥29॥
Through this practice, the immutable self is revealed and all obstacles (antaraya) are removed. ||29||
व्याधि स्त्यान संशय प्रमादाअलस्याविरति भ्रान्तिदर्शनालब्धभूमिकत्वानवस्थितत्वानि चित्तविक्षेपाः ते अन्तरायाः ॥३०॥
In a more contemporary source - an explanation of the om mani padme hum mantra, written by the Dalai Lama, we see a similar definition of the word Om.
by His Holiness Tenzin Gyatso The Fourteenth Dalai Lama of Tibet
It is very good to recite the mantra OM MANI PADME HUM, but while
you are doing it, you should be thinking on its meaning, for the
meaning of the six syllables is great and vast. The first, OM, is
composed of three pure letters, A, U, and M. These symbolize the
practitioner's impure body, speech, and mind; they also symbolize the
pure exalted body, speech and mind of a Buddha."