diph·thong 
(dĭfthông′, -thŏng′, dĭp-)
(dĭfthông′, -thŏng′, dĭp-)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.
[Middle English diptonge, from Old French diptongue, from Late Latin diphthongus, from Greek diphthongos : di-, two; see DI-1 + phthongos, sound.]
diph·thongal adj.