open-mouth sound; pronunciation without a medial glide
In Chinese phonology, a syllable that lacks a medial glide (such as -i-, -u-, -ü-) between the initial consonant and the central vowel. Technical term used in historical Chinese phonetics and Sino-Japanese studies.
In Middle Chinese, 開口音 refers to syllables without a medial glide.
long 'o' from 'a' + 'u' combination
In Japanese historical phonology, the long /oː/ vowel that results from the contraction of /a/ and /u/ sounds, such as in the development of certain Sino-Japanese readings. Rare, specialized term.
開口音 is used in Japanese historical phonology to explain the long 'o' sound that arose from 'a' + 'u'.