Aksara or Akshara in Sanskrit means, as a noun - letter, syllable, letter of alphabet, final beatitude religious austerity, sound, sacrifice, water, alphabet, syllable Aum, vowel, word or mark etched on metal or stone, and as an adjective – fixed, imperishable, unalterable, immutable.
In the Brahmanas and the Upanishads, Aksara means syllable and letter Aum, the Ekaksara, and imperishable, indestructible, immutable, undying, un-decaying, all in reference to the individual self and the Supreme Self, Brahman.
Read more about Aksara: Aksara in Indian Writing System, Aksara's Relevance in The Vedas and The Upanishads, Aksara's Relevance in The Bhagavad Gita, Aksara's Relevance in Hindu Philosophy