In phonology and linguistics, a phoneme () is a unit of sound that can distinguish one word from another in a particular language.

For example, in most dialects of English, with the notable exception of the West Midlands and the north-west of England, the sound patterns (sin) and (sing) are two separate words that are distinguished by the substitution of one phoneme, /n/, for another phoneme, /ŋ/. Two words like this that differ in meaning through the contrast of a single phoneme form a minimal pair. If, in another language, any two sequences differing only by pronunciation of the final sounds [n] or [ŋ] are perceived as being the same in meaning, then these two sounds are interpreted as phonetic variants of a single phoneme in that language.

Phonemes that are established by the use of minimal pairs, such as tap vs tab or pat vs bat, are written between slashes: /p/, /b/. To show pronunciation, linguists use square brackets: [pʰ] (indicating an aspirated p in pat).

There are differing views as to exactly what phonemes are and how a given language should be analyzed in phonemic (or phonematic) terms. However, a phoneme is generally regarded as an abstraction of a set (or equivalence class) of speech sounds (phones) that are perceived as equivalent to each other in a given language. For example, the English k sounds in the words kill and skill are not identical (as described below), but they are distributional variants of a single phoneme /k/. Speech sounds that differ but do not create a meaningful change in the word are known as allophones of the same phoneme. Allophonic variation may be conditioned, in which case a certain phoneme is realized as a certain allophone in particular phonological environments, or it may otherwise be free, and may vary by speaker or by dialect. Therefore, phonemes are often considered to constitute an abstract underlying representation for segments of words, while speech sounds make up the corresponding phonetic realization, or the surface form.