Real Dictionary

triphthong in American English
a complex vowel sound involving three continuous vowel sounds in one syllable (Ex.: fire, as in the British pronunciation, IPA [ faɪə])
noun: a monosyllabic speech-sound sequence perceived as being made up of three differing vowel qualities, as the pronunciation of our, esp. in r-dropping dialects

triphthong in British English
noun: a composite vowel sound during the articulation of which the vocal organs move from one position through a second, ending in a third