1. two vowels as one syllable: a complex vowel sound in which the first vowel gradually moves toward a second vowel so that both vowels form one syllable, e.g. "a" and "i" in "rail"
2. joined letters: a character formed by joining the two letters "a" and "e" as "æ" or the two letters "o" and "e" as "œ"
[15th century. Via French< Latin diphthongus "two sounds" < Greek phthoggos "sound"]
