In human phonology, a phoneme (from the , phōnēma, "a sound uttered") is the smallest linguistically distinctive unit of sound. Phonemes carry no semantic content themselves. In theoretical terms, phonemes are not the physical segments themselves, but cognitive abstractions or categorizations of them. A morpheme is the smallest structural unit with meaning.

In effect, a phoneme is a group of slightly different sounds which are all perceived to have the same function by speakers of the language in question. An example of a phoneme is the /k/ sound in the words kit and skill. (In transcription, phonemes are placed between slashes, as here.) Even though most native speakers don't notice, in most dialects, the ks in each of these words are actually pronounced differently: they are different speech sounds, or phones (which, in transcription, are placed in square brackets). In our example, the /k/ in kit is aspirated, 1, while the /k/ in skill is not, 2. The reason why these different sounds are nonetheless considered to belong to the same phoneme in English is that if an English-speaker used one instead of the other, the meaning of the word would not change: saying 3 in skill might sound odd, but the word would still be recognized. By contrast, some other sounds could be substituted which would cause a change in meaning, producing words like still (substituting /t/), spill (substituting /p/) and swill (substituting /w/). These other sounds (/t/, /p/ and /w/) are, in English, different phonemes. In some languages, however, 4 and 5 are different phonemes, and are perceived as such by the speakers of those languages. Thus, in Icelandic, /kʰ/ is the first sound of kátur 'cheerful', while /k/ is the first sound of gátur 'riddles'.

In many languages, each letter in the spelling system represents one phoneme. However, in English spelling there is a very poor match between spelling and phonemes. For example, the two letters sh represent the single phoneme /ʃ/, while the letters k and c can both represent the phoneme /k/ (as in kit and cat).

Phones that belong to the same phoneme, such as 6 and 7 for English /t/, are called allophones. A common test to determine whether two phones are allophones or separate phonemes relies on finding minimal pairs: words that differ by only the phones in question. For example, the words tip and dip illustrate that 8 and 9 are separate phonemes, /t/ and /d/, in English, whereas the lack of such a contrast in Korean (/tʰata/ is pronounced 10, for example) indicates that in this language they are allophones of a phoneme /t/.