A phoneme is a basic element of a spoken language or dialect, from which words in that language or dialect are analyzed as being built up. The phoneme is defined by the International Phonetic Association as "the smallest segmental unit of sound employed to form meaningful contrasts between utterances".

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 perceived as the same sound by speakers of the language, and are therefore both considered to represent a single phoneme, /k/. Different speech sounds representing the same phoneme are known as allophones. Thus phonemes are often considered to provide an underlying representation for words, while speech sounds make up the corresponding surface form.