A term invented by the American phonetician Kenneth Pike (1912-2000) to help distinguish between the phonetic and the phonological notions of vowel. Phonetically, a vowel is defined as a sound lacking any closure or narrowing sufficient to produce audible friction. Phonologically, it is a unit which functions at the centre of syllable. In cases such as [l], [r], [w] and [j], however, these criteria do not coincide: these sounds are phonetically vowel-like, but their function is consonantal. To avoid possible confusion, Pike proposed the term ‘vocoid’ for sounds which are characterized by a phonetic definition such as the above; the term ‘vowel’ is the reserved for the phonological sense. Its opposite is contoid. Since the 1980s, the term has become fashionable in feature geometry models of phonology, where it is often used to designate one of the two chief classes of segments (the other being consonants).