Onbin is a linguistic term that refers to sound changes made for easier pronunciation.
In the history of Japanese, verb conjugations changed due to euphonic changes.
i-onbin; euphonic change to 'i'
u-onbin; euphonic change to 'u'
euphonic change form
Compound of 音 (on, 'sound') and 便 (bin, 'convenience'), literally 'sound convenience'. The term was coined in Japanese linguistics to describe sound changes that facilitate pronunciation.