1. The smallest unit of speech that distinguishes one utterance from another in all of the variations that it displays in the speech of a single person or particular dialect as the result of modifying influences (as neighboring sounds and stress).
2. The smallest sound unit which, in terms of phonetic sequences of sound, controls meaning.
3. The basic phonologic unit of a spoken language, identified in terms of a particular vowel or consonant.
4. An auditory hallucination of voices and spoken words.