A phoneme is a basic unit of a language's phonology, which is combined with other phonemes to form meaningful units such as words or morphemes. The phoneme can be described as "the smallest segmental unit of sound employed to form meaningful contrasts between utterances". In this way the difference in meaning between the English words kill and kiss is a result of the exchange of the phoneme /l/ for the phoneme /s/. Two words that differ in meaning through a contrast of a single phoneme are called minimal pairs.
Within linguistics there are differing views as to exactly what phonemes are and how a given language should be analyzed in phonemic 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 representing the same phoneme are known as allophones, and such 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 words, while speech sounds make up the corresponding phonetic realization, or surface form.
Read more about Phoneme: Notation, Assignment of Speech Sounds To Phonemes, Other Features With Phonemic Status, Distribution of Allophones, Background and Related Ideas, Restrictions On Occurrence, Biuniqueness, Neutralization and Archiphonemes, Morphophonemes, Numbers of Phonemes in Different Languages, Correspondence Between Letters and Phonemes, Phonemes in Sign Languages