diphthong

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A complex speech sound or glide that begins with one vowel and gradually changes to another vowel within the same syllable, as (oi) in boil or (ī) in fine.

Century Dictionary and Cyclopedia

1. n. A coalition or union of two vowels pronounced in one syllable. In uttering a proper diphthong both vowels are pronounced; the sound is not simple, but the two sounds are so blended as to be considered as forming one syllable, as in joy, noise, bound, out. An “improper” diphthong is not a diphthong at all, being merely a collocation of two or more vowels in the same syllable, of which only one is sounded, as ea in breach, eo in people, ai in rain, eau in beau. (See digraph.) In Greek grammar, a proper diphthong is a diphthong the first vowel of which is short; an improper diphthong, a diphthong the first vowel of which is long. The proper diphthongs are αι, ει, οι, αυ, ευ, ου; the improper, α%26ι, ηι, ωι (commonly written ᾳ,ῃ,ῳ: see iota subscript, under subscript), ηυ, ωυ. An improper diphthong not usually distinguished as such is α%26υ, as in να%26υ%21ς, Epic νηυ%21ς. Some include υτ in this class, and some limit the term to ᾳ, ῃ, ῳ.
2. To sound as a diphthong; diphthongize.

Wiktionary

1. n. A complex vowel sound that begins with the sound of one vowel and ends with the sound of another vowel, in the same syllable.
2. n. A vowel digraph or ligature.

GNU Webster's 1913

1. n. A coalition or union of two vowel sounds pronounced in one syllable; as, ou in out, oi in noise; -- called a proper diphthong.
2. n. A vowel digraph; a union of two vowels in the same syllable, only one of them being sounded; as, ai in rain, eo in people; -- called an improper diphthong.
3. v. To form or pronounce as a diphthong; diphthongize.

WordNet 3.0

1. n. a vowel sound that starts near the articulatory position for one vowel and moves toward the position for another