Phoneme
Etymology
Through French phonème from Greek phōnēma, "a sound, an utterance," from phōnein ("to make a sound"), from phōnē ("voice, sound"). The abstract linguistic sense took shape in the late nineteenth century. A phoneme is not a physical sound so much as a category of sound that speakers of a language treat as the same.
noun · /FOH-neem/
The smallest unit of sound that can distinguish one word from another in a language.
What it names
The atoms of spoken language — the reason swapping a single sound turns "pat" into "bat," and why what counts as one sound differs from language to language.
Example
The difference between the English /r/ and /l/ is two phonemes — a distinction some languages don’t make at all.
Related idea
A phoneme is the smallest unit of sound; a morpheme is the smallest unit of meaning. The word "cats" has four phonemes but two morphemes — "cat" and the plural "-s."