Kan-on

 is one of the sources of pronunciation of Japanese kanji. They were borrowed during the Tang dynasty (7th to 9th century), introduced by, among others, envoys from Japanese missions to Tang China. This period corresponds with the Japanese Nara period. Not to be confused with Tō-on "Tang sound", which actually refers to later phonetic loans.
"Kan-on" is based on the central Chang'an pronunciation. The name "Kan" could refer to the Han dynasty, which also had Chang'an as its capital city. Furthermore, "Kan" has also become a description for all things Chinese, e.g., Kanji ('Chinese characters').
Kan'on partly displaced the earlier "go'on", which were "just imitations of Korean imitations, but "Kan-on" were imitations of the real things."
A minority of characters never had their "Kan-on" transmitted to Japan; their "Kan-on" are sometimes reconstructed in Japanese dictionaries although not specifically marked as such. A few dictionaries go as far as to discard attested "Kan-on" in favour of more systematic pronunciations.