atomic unit
Definition - The smallest language unit that higher-level language units are constructed from.

Example -
The atomic unit called the phoneme is the smallest unit of meaningful sound. Some others are: graphemes, lexemes, morphemes, sememes, and tonemes

Etymology -
The word atomic derives via Latin from the Greek atomos, uncut (from a, not + tomos, a cutting). The key idea is that an atom is something that cannot be broken up into smaller units.

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