Phoneme: A basic sound unit in a language. While the pronunciation of a phoneme may vary — the p sound, for instance, is aspirated in pot but not in spot — each phoneme is a contrastive sound unit, which means that substituting one phoneme in a word for another produces a change in meaning (or results in a nonword). Change the f sound in the English word phaser to an l sound, and the result is laser; change it to a k sound, and a possible — but nonexistent — word results (kaser). Phonemes may thus be seen as the building blocks of words. Each basic sound unit can be distinctly articulated, as often happens when a child is learning to read (phoneme: f-ō-n-ē-m), and yet one must move smoothly from one phoneme to the next to enunciate a word properly.
An alphabetic letter is not equivalent to a phoneme. For instance, f and ph represent a single phoneme in English, as they always signify the same sound. The letter c may signify the phonemic sound represented by the letters k or s, as in the words cool and cereal. Ou, when followed by gh, itself represents at least five phonemes, as in the following series of words: dough, cough, through, bough, slough. Furthermore, many letters, when followed by the letter h in our linguistic system, produce entirely different phonemes than those we most commonly associate with the letters themselves: church, thanks, rough, trophy.
The number of phonemes in any linguistic system is limited. English is often said to have about forty-five phonemes; Arabic thirty-four; Italian twenty-seven; and Rotokas, a language spoken in Papua New Guinea, only eleven.