A in Buddhism

Mahayana Buddhism invested the phoneme with mystical significance, associated with the doctrine of emptiness.

Thus, for example, svabhāva, “with essence,” can be changed to asvabhāva, “without essence.”[3] The letter also came to signify the Mahayana teaching of Prajñāpāramitā (the Perfection of Wisdom).

One of the Prajñāpāramitā sutras is the short The Perfection of Wisdom Mother in One Syllable (ekākṣarīmātāprajñāpāramitā).

[1] The Pañcaviṃśatisāhasrikā Prajñāpāramita Sutra (The Perfection of Wisdom in 25,000 lines) contains a method for meditating on the letters of the Arapacana alphabet (an alphabet associated with Karosthi) as part of a practice which is termed "entrance into the door of the dhāraṇīs, the entrance into the exposition of the letters".

"[4] Specifically, the sutra states that A "is a door to the insight that all dharmas are unproduced from the very beginning (akāro mukhaḥ sarvadharmāṇāṃ ādyanutpannavāt).

[1][14] This teaching and practice is one of the most fundamental elements of the Shingon system taught by the Japanese tantric master Kūkai (774-835).