the GNU version of the Collaborative International Dictionary of English
adj. Relating or belonging to a diphthong; having the nature of a diphthong.
The Century Dictionary and Cyclopedia
Belonging to a diphthong; consisting of two vowel-sounds pronounced in one syllable.
Word Usage
"The distinctions might not be easy to find, but these 'higher order' signs are generally assumed to be either different diphthongal versions, or those with an aspirated consonant."