A phoneme (//) is one of the units of sound (or gesture in the case of sign languages, see chereme) that distinguish one word from another in a particular language. For example, in most dialects of English, the sound patterns // (thumb) and // (dumb) are two separate words distinguished by the substitution of one phoneme, /θ/, for another phoneme, /d/. (Two words like this that differ in meaning through a contrast of a single phoneme form what is called a minimal pair). In many other languages these would be interpreted as exactly the same set of phonemes (i.e. /θ/ and /d/ would be considered the same).
In linguistics, phonemes (usually established by the use of minimal pairs, such as kill vs kiss or pat vs bat) are written between slashes, e.g. /p/. To show pronunciation more precisely linguists use square brackets, for example (indicating an aspirated p).
Within linguistics 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) which are perceived as equivalent to each other in a given language. For example, in English, the k sounds in the words kit and skill are not identical (as described below), but they are distributional variants of a single phoneme /k/. Different speech sounds that are realizations of the same phoneme are known as allophones. Allophonic variation may be conditioned, in which case a certain phoneme is realized as a certain allophone in particular phonological environments, or it may be free in which case it may vary randomly. In this way, phonemes are often considered to constitute an abstract underlying representation for segments of words, while speech sounds make up the corresponding phonetic realization, or surface form.
Phonemes are conventionally placed between slashes in transcription, whereas speech sounds (phones) are placed between square brackets. Thus /pʊʃ/ represents a sequence of three phonemes /p/, /ʊ/, /ʃ/ (the word push in standard English), while represents the phonetic sequence of sounds (aspirated p), , (the usual pronunciation of push). This is not to be confused with the similar convention of the use of angle brackets to enclose the units of orthography, namely graphemes; for example, ⟨f⟩ represents the written letter (grapheme) f.
The symbols used for particular phonemes are often taken from the International Phonetic Alphabet (IPA), the same set of symbols that are most commonly used for phones. (For computer typing purposes, systems such as X-SAMPA and Kirshenbaum exist to represent IPA symbols using only ASCII characters.) However, descriptions of particular languages may use different conventional symbols to represent the phonemes of those languages. For languages whose writing systems employ the phonemic principle, ordinary letters may be used to denote phonemes, although this approach is often hampered by the complexity of the relationship between orthography and pronunciation (see Correspondence between letters and phonemes below).
A phoneme is a sound or a group of different sounds perceived to have the same function by speakers of the language or dialect in question. An example is the English phoneme /k/, which occurs in words such as cat, kit, scat, skit. Although most native speakers do not notice this, in most English dialects the "c/k" sounds in these words are not identical: in kit (help·info) the sound is aspirated, while in skill (help·info) it is unaspirated. The words therefore contain different speech sounds, or phones, transcribed for the aspirated form, for the unaspirated one. These different sounds are nonetheless considered to belong to the same phoneme, because if a speaker used one instead of the other, the meaning of the word would not change: using the aspirated form in skill might sound odd, but the word would still be recognized. By contrast, some other sounds would cause a change in meaning if substituted: for example, substitution of the sound would produce the different word still, and that sound must therefore be considered to represent a different phoneme (the phoneme /t/).
The above shows that in English and are allophones of a single phoneme /k/. In some languages, however, and are perceived by native speakers as different sounds, and substituting one for the other can change the meaning of a word; this means that in those languages, the two sounds represent different phonemes. For example, in Icelandic, is the first sound of kátur meaning "cheerful", while is the first sound of gátur meaning "riddles". Icelandic therefore has two separate phonemes /kʰ/ and /k/.