Phoneme
A phoneme is the smallest unit of language that can distinguish words or syllables, and is of interest in the subfield of linguistics called phonology. In oral languages, phonemes are regarded as the individual sounds, or groups of sounds, of the language, corresponding very roughly to the sounds of the letters of an alphabet. 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 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, and not a phone, is that changing it to another phoneme may yield a different word. For instance, /mat/ ("mat") and /mad/ ("mad") are different words of English, distinguished by the /t/ and /d/ phonemes. 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 grammar of the language.