What is "aksara"

Aksara (also akshara, Devanagari अक्षर, IAST akṣara) is a Sanskrit term translating to "imperishable, indestructible, fixed, immutable" (i.e. from a- "not" and kṣar- "melt away, perish").

It has two main fields of application, in Sanskrit grammatical tradition ( śikṣā) and in Vedanta philosophy. The uniting aspect of these uses is the mystical view of language, or shabda, in Hindu tradition, and especially the notion of the syllable as a kind of immutable (or "atomic") substance of both language and truth, most prominently, of course, the mystical syllable Aum, which is given the name of ekākṣara (i.e. eka-akṣara), which can be translated as both "the sole imperishable thing" and as "a single syllable". In the explicitly monotheistic tradition of Bhakti yoga, both akṣara and aum become seen as a symbol or name of God.