A viseme is a representational unit used to classify speech sounds in the visual domain. The term viseme was introduced based on the interpretation of the phoneme as a basic unit of speech in the acoustic/auditory domain, (Fisher, 1968). This is, however, at variance with the accepted definition of the phoneme as the smallest structural unit that distinguishes meaning within a given language—as a cognitive abstraction that is not bound to any sensory modality.
A "viseme" describes the particular facial and oral positions and movements that occur alongside the voicing of phonemes. The analogous term for the acoustic reflection of a phoneme would be "audieme", but this is not in use.
Phonemes and visemes do not always share a one-to-one correspondence; often, several phonemes share the same viseme. In other words, several phonemes look the same on the face when produced, such as /k/, /ɡ/, /ŋ/, (viseme: /k/), or /t͡ʃ/, /ʃ/, /d͡ʒ/, /ʒ/ (viseme: /ch/). However, there could be differences in timing and duration during actual speech in terms of the visual 'signature' of a given gesture that can not be captured with a single photograph. Conversely, some sounds which are hard to distinguish acoustically are clearly distinguished by the face (Chen 2001). For example, acoustically speaking English /l/ and /r/ could be quite similar (especially in clusters, such as 'grass' vs. 'glass'). Yet visual information can show a clear contrast. This is demonstrated by the more frequent mishearing of words on the telephone than in person. Some linguists have argued that speech is best understood as bimodal (aural and visual), and comprehension can be compromised if one of these two domains is absent (McGurk and MacDonald 1976). The comprehension of speech by visemes alone is known as speechreading or "lip reading".
Applications for the study of visemes includes speech processing, speech recognition and computer facial animation.