“The smallest distinct sound unit in a given language: e.g. tip in English realizes three successive phonemes realized in spelling by the letters t, i, and p.
Detailed definitions have varied from one theory to another, But, in general, two words are composed of different phonemes only if they differ phonetically in ways that are found to make a difference in meaning. Thus in English i and a are difference phonemes since, for example, tip does not mean the same as tap, nor pit the same as pat. The individual phonemes are then the smallest units in each word that distinguish meanings and, in addition, are realized over distinct time spans. By the same criterion, i and a are single phonemes since they cannot be analyzed into smaller units meeting the criterion, each with its own time span.
Thence phonemic; e.g. a phonemic transcription of a word, etc. is its representation as a sequence or other combination of phonemes.”
Excerpted from: Matthews, P.H. The Oxford Concise Dictionary of Linguistics. New York: Oxford University Press, 2014.