An akshara (Sanskrit: अक्षर, romanized: akṣara, lit. 'imperishable, indestructible, fixed, immutable') is a consonant letter together with any vowel diacritics in a Brahmic script. It is a term used in the traditional grammar of the Sanskrit language and in the Vedanta school of Indian philosophy.

The term derives from अ, a- "not" and क्षर्, kṣar- "melt away, perish". The uniting aspect of its use 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, 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".

The akshara is the unit of graphemic symbols in the Brahmic scripts. An akshara is more a syllable-like unit for writing which requires the knowledge of syllables and the matra, i.e. the measure of prosodic marking. In writing it prototypically stands for CV, CVV, CCV, CCVV, CCCV, CCCVV, V and VV where "C" stands for a consonant, "V" for a vowel and "VV" for a long vowel. It is usually a sub-syllabic representation which stands for onset, onset plus nucleus and nucleus alone; the coda part of a syllable goes into the next akshara in a word.

Its nature favours the phonological mediation i.e. the non-lexical strategy of reading, which may be interpreted in stages such as the "Visual Analysis System" and proceeding to the "Aksara Recognition System" and then to the "Aksara sound Conversion System" and the "Phonological Assembly System" before ending with the "Response Buffer" prior to reading aloud.^{[clarification needed][dubious – discuss]}

As part of basic instructions of Shiksha and Sanskrit grammar, it is explained that among the Word-entities, both, Aksara and Brahman stand out as especially important because both refer to a special form of ritual word. In the Brahmanas and the Upanishads both come to mean the Absolute. Aksara is a part of the Sacred Word "Om" and possesses a unique and intensified power and dignity. In the Upanishads, in a condensed and intensified form, it signifies the transcendent Principle of all that exists. Its significance is derived from its role as a "syllable", the essential and embryonic core of speech.

The eighth brahmana of the Brihadaranyaka Upanishad focuses on the concept of the imperishable (akshara). Verse 3.8.8-9 describe akshara having certain qualities. It is neither physical nor subtle, and it is not defined by physical characteristics such as size, length, or physical traits like blood or fat. It exists beyond shadows, darkness, air, and space, untouched by sensory experiences like taste, smell, sight, or hearing. It does not possess attributes like speech, mind, energy, breath, and form. Akshara is immeasurable, existing without internal or external elements, and is neither a consumer nor consumed. Akshara governs the universe and influences various aspects of existence including the sun, moon, earth, sky, time, rivers, human behavior, and the dependence of gods and ancestors on ritual offerings. Verse 3.8.10 emphasizes the significance of understanding this imperishable:

Without knowing this imperishable, Gargi, even if a man were to make offerings, to offer sacrifices, and to perform austerities in this world for many thousands of years, all that would come to naught. Pitiful is the man, Gargi, who departs from this world without knowing this imperishable.

— Brihadaranyaka Upanishad, Chapter 3, Brahmanam 8, Hymn 10