The difficulty of giving such a definition lies in the fact that the Phoneme has several aspects and functions.
There are several conceptions of the phoneme among Russian and foreign linguists and none of them is satisfactory from all points of view. Linguists have not arrived to the definition of the phoneme acceptable to all. Allophones are sounds that cannot distinguish words in a definite language, they occur only in certain positions or in combination with certain sounds, they are realizations of one and the same phoneme. These really pronounced variants are called allophones /vaɪn/ - /waɪn/. In actual speech sounds pronounced by speakers are variants of the phonemes existing in the language. These sound types are called phonemes, in English there are 20 vowel phonemes and 24 consonant phonemes (6 vowels, 35 consonants – Russian).
Every language has a limited number of sound types or phonemes, they are shared by all the native speakers of the language and they are linguistically important because they distinguish words. The first type of sounds which is phonologically important is phonemes. pill – aspiration, spill – less aspiration, slipper – p – partially voiced. They can't differentiate words and their substitution does not interfere with communication it only causes a foreign accent, e.g. At the same time there are sounds which partially differ in articulation and acoustic value, but they don't differ phonologically. Their interchange changes the meaning of the words and interferes with communication. In real speech we pronounce a great variety of sounds, some of the sounds help to differentiate words, because they are different phonologically. Unlike phonetics which studies sounds as articulatory and acoustic units. Phonology is a branch of Phonetics which investigates sounds of speech, as units which serve people for communicative purposes. The Phoneme, its Definition, Aspects and Functions