Diph·thong n.  Orthoëpy (a) A coalition or union of two vowel sounds pronounced in one syllable; as, ou in out, oi in noise; -- called a proper diphthong. (b) 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.


 Im·prop·er a.
 1. Not proper; not suitable; not fitted to the circumstances, design, or end; unfit; not becoming; incongruous; inappropriate; indecent; as, an improper medicine; improper thought, behavior, language, dress.
 Follow'd his enemy king, and did him service,
 Improper for a slave.   --Shak.
 And to their proper operation still,
 Ascribe all Good; to their improper, Ill.   --Pope.
 2. Not peculiar or appropriate to individuals; general; common. [Obs.]
    Not to be adorned with any art but such improper ones as nature is said to bestow, as singing and poetry.   --J. Fletcher.
 3. Not according to facts; inaccurate; erroneous.
 Improper diphthong. See under Diphthong.
 Improper feud, an original feud, not earned by military service. --Mozley & W.
 Improper fraction. See under Fraction.