A phoneme is the smallest distinctive sound unit in a language. Changing a phoneme changes the meaning of a word.

In English, the sounds /b/ and /p/ are different phonemes because “bat” and “pat” mean different things. Languages differ in which sound contrasts they treat as distinctive. Infants are initially sensitive to a wide range of phonetic distinctions; by the end of the first year they become more specialized for the phonemes of the language they hear most. The process of categorical perception assigns continuously varying acoustic signals to discrete phoneme categories.

Phonemes are abstract categories rather than fixed acoustic events. The same phoneme is produced differently depending on surrounding sounds, speaker, and speaking rate. Listeners nevertheless recover the intended phonemes with high accuracy under ordinary conditions. Disorders of phonological processing can impair reading and spoken language.

The difference between one word and another often comes down to a single phoneme.