pentaphthong

From penta- +‎ -phthong, by analogy with diphthong, triphthong.

pentaphthong (plural pentaphthongs)

1. (phonetics, rare) A sequence of five vowels pronounced as, or almost as, a single syllable.
   • 1866, Caleb Bates Josselyn, Elements of Pronunciation […], page 53:

     Fifty-six diphthongal combinations may be made from the eight vowel elements, four of which, though, we find to be necessarily triphthongs, two tetraphthongs, and one a pentaphthong.

   • 1992 February 4, Graham Toal, “Vowel Raising Before Nasals”, in bit.listserv.words-l‎ (Usenet):

     Try saying the Dutch word Leeuwen some time. At least a pentaphthong...

   • 1993 December 23, Bob Simmonds, “Seamed phthongs”, in bit.listserv.words-l‎ (Usenet):

     Someone once claimed that Pres. Coolidge ( a Vermonter) pronounced the vowel in "cow" as a pentaphthong.

   • 2008 June 17, Ruud Harmsen, “British English "Joshua is"”, in sci.lang‎ (Usenet):

     Phonetically, [j] is meaningless, it's the same as an [i]. So what we have here is a pentaphthong: ui@iu.