u-onbin; euphonic change to 'u'
Linguistics term for a historical sound shift where certain mora (ku, gu, hi, bi, mi) become 'u', common in Western Japanese dialects and in classical verb forms like て形.
The 'ite' in 'kaite' is a result of u-onbin.
Arigataku becoming arigatou is an example of u-onbin.
Compound of ウ (u) + 音便 (onbin, 'euphonic change'). The term describes a specific type of sound shift in Japanese historical linguistics.