onbin 音便

In Japanese, onbin 音便, often translated as "euphonic change," refers to four types of changes in pronunciation that affect, among other things, the ren'youkei 連用形 conjugation of verbs. Such changes exist to make the words easier to pronounce. They are:

1. u-onbin
   ウ音便
   Changes a kana to u う.
   Example: omohi-te 思ひて
   Becomes: omoute 思うて.
2. i-onbin
   イ音便
   Changes a kana to i い.
   Example: kaki-te 書きて
   Becomes: kaite 書いて.
3. hatsuonbin
   撥音便
   Changes a kana to n ん. This n ん is called hatsuon.
   Example: shini-te 死にて
   Becomes: shinde 死んで.
4. sokuonbin
   促音便
   Changes a kana to the small tsu っ, which represents a "geminate consonant," sokuon 促音.
   Example: tori-te 取りて
   Becomes: totte 取って.

See onbinkei 音便形 for a more complete list of uses.

According to the dictionary Nihon Kokugo Daijiten 日本国語大辞典, Motoori Norinaga 本居宣長 (1730–1801) was the scholar that introduced the onbin terms to Japanese grammar. When he did so, he also included the rendaku 連濁 in the definition of onbin.^{[音便 - kotobank.jp, accessed 2019-11-10]}