Definition of subjunctive vowel in US English:

subjunctive vowel

noun

  • 1The second vowel of a diphthong.

  • 2The vowel by which the subjunctive is distinguished from the indicative.

Origin

Mid 18th century; earliest use found in Thomas Nugent (c1700–1772), writer and traveller. After post-classical Latin vocalis subiunctiva, Hellenistic Greek ϕωνῆεν ὑποτακτικόν.