pho·neme 
(fōnēm′)
(fōnēm′)n.
The smallest phonetic unit in a language that is capable of conveying a distinction in meaning, as the m of mat and the b of bat in English.
[French phonème, from Greek phōnēma, phōnēmat-, utterance, sound produced, from phōnein, to produce a sound, from phōnē, sound, voice; see bhā-2 in the Appendix of Indo-European roots.]