What is "yoon"

is a feature of the Japanese language in which a mora is formed with an added sound, i.e., palatalized.

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 Historical kana orthography, 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.

Yoon is the eighth most common family name in Korea. The name is sometimes also transliterated as Yun, Yune, or Youn. The Hanja character 尹 is defined as "eldest" in Korean which is different from the Chinese definition. The use of Hanja for personal names has become less common in Korea.

The Chinese character 尹 is also used by the following families Yǐn (surname) in China and as Doãn in Vietnam. However, the Korean surname Yoon(Yun) has no relations with the Yin of China and Doãn of Vietnam.