in linguistics, smallest unit of speech distinguishing one word (or word element) from another, as the sound p in “tap,” which separates that word from “tab,” “tag,” and “tan.” A phoneme may have more than one variant, called an allophone (q.v.), which functions as a single sound; for example, the p's of “pat,” “spat,” and “tap” differ slightly phonetically, but that difference, determined by context, has no significance in English. In some languages, where the variant sounds of p can change meaning, they are classified as separate phonemes—e.g., in Thai (Thai language) the aspirated p (pronounced with an accompanying puff of air) and unaspirated p are distinguished one from the other.

Phonemes are based on spoken language and may be recorded with special symbols, like those of the International Phonetic Alphabet. In transcription, linguists conventionally place symbols for phonemes between slash marks: /p/. The term phoneme is usually restricted to vowels and consonants, but some linguists extend its application to cover phonologically relevant differences of pitch, stress, and rhythm. Nowadays the phoneme has a less central place in phonological theory than it used to have, especially in American linguistics. Many linguists regard the phoneme as a set of simultaneous distinctive features, rather than as an unanalyzable unit.