An ELT Glossary : Phoneme
A phoneme is a speech sound which, in a specific language, makes a difference in meaning. For example, in English the choice of the initial sound in words such as lip, rip, tip, pip, nip etc changes the meaning of the word. Thus the consonants involved - /l r t p n/ - are considered to be phonemes of English.
Phonemes are, however, language specific. For example, in some South East Asian languages (eg Korean) /l/ and /r/ are not phonemes but allophones - they both occur in the language but as variants of the same phoneme, and are not necessarily perceived as "different" by native speakers. Conversely the phoneme /p/ in English also has two allophones - aspirated as in "pin" [pʰ] as in "pin" [pʰɪn] and unaspirated as in "spin". Native speakers of English won't perceive a difference, although they use the sounds automatically. However, in Korean, the aspiration distinction does make a difference in meaning - [p͈ul] means "horn", while [pʰul] means "grass". In Korean, these two sounds are therefore separate phonemes.