Yōon or Youon (拗音, contracted word or diphthong) is a feature of the Japanese language in which a mora is formed with an added [j] sound.
Yōon are represented in hiragana using a kana ending in i, such as き (ki), plus a smaller-than-usual version of one of the three y kana, ya, yu or yo. For example kyō, "today", is written きょう, using a small version of the yo kana, よ. Contrast this with kiyō, "skillful", which is written きよう, with a full-sized yo kana. In the past, in historical kana usage, yōon were not distinguished with the smaller kana, and had to be determined by context.
In earlier Japanese, yōon could also be formed with the kana wa, wi, we, and wo; for example, くゎ/クヮ kwa, くゐ/クヰ kwi, くゑ/クヱ kwe, くを/クヲ kwo. Although obsolete in modern Japanese, kwa and kwi can still be found in several of the Ryukyuan languages today, while kwe is formed with the digraph くぇ. Instead of the kana き, these are formed with the kana for ku, く/ク.
Other Representations 
In Japanese Braille, Yōon is indicated with one of the yōon, yōon+dakuten, or yōon+handakuten prefixes.
Unlike in kana, Braille yōon is prefixed to the -a/-u/-o syllables, rather than the -i, e.g. きゅ (kyu) = yōon + ku: ⠈⠩. Likewise, the -w- syllables are indicated by a prefix of the -a/-i/-e/-o syllables, rather than an -u syllable, e.g. くぁ / くゎ (kwa) = -w- + ka: ⠢⠡.