A phoneme is a unit of language that can distinguish one word from another as different meaningful units, such as /b/ versus /m/ in English bat and mat. It is often considered the smallest unit of language that can serve that distinguishing purpose, but others have been proposed. It is of interest in the subfield of linguistics called phonology.
In spoken language, phonemes are regarded as the individual 'sounds' of the language, corresponding very roughly to the sounds of the letters of an alphabet—though a phoneme may also be in fact a small group of consecutive sounds.
The concept of the phoneme is used in understanding sign languages as well.
The existence of phonemes is generally assumed in many fields of linguistics and education, but it has been rejected as a true unit of analysis in mainstream phonology since the publication of Chomsky and Halle's The Sound Pattern of English in 1968.
Phonemes and other phonological segments
Phonemes are not the smallest level of sound classification. The sound inventory of a language is the set of phones of that language. However, phones do not distinguish meaning in themselves. What makes a phoneme a phoneme, rather than a phone, is that changing it to another phoneme may yield a different word. For instance, /mat/ ("mat") and /mad/ ("mad") are different words in English. They are distinguished by only one different phoneme: /t/ versus /d/ (put more specifically, it is the opposition between voiceless and a voiced dental plosive in the first versus the second word). This difference is called phonemic. In addition, word pairs such as "bat/mat" or "mat/mad" in English are called minimal pairs, because they are only distinguished by one phoneme.
On the other hand, the phoneme /t/ can be pronounced as one of several distinct phones ([t] (unaspirated) and [th] (aspirated t)), called the allophones of /t/. The choice of allophone depends on the context surrounding the /t/ and the phonological rules in the language ([th], for example, occurs initially in English stressed syllables, e.g. [ə.thæk] attack). As a result, changing between allophones (such as saying [tom] vs [thom]) would not yield a new word, as above, but instead would be a violation of the phonological constraints (phonotactics) of the language.