Tō-on in the context of On reading

A single kanji might have multiple on'yomi pronunciations, reflecting the Chinese pronunciations from different time periods or regions. On'yomi pronunciations are generally classified into go-on, kan-on, tō-on and kan'yō-on, roughly based on when they were borrowed from Chinese during the peaks of the Sinosphere.

Kan-on or kan'on (漢音; Japanese pronunciation: [kaꜜɰ̃.oɴ] or [kaɰ̃.oɴ], "Han sound") are Japanese kanji readings borrowed from Chinese during the Tang dynasty, from the 7th to the 9th centuries; a period which corresponds to the Japanese Nara period. They were introduced by, among others, envoys from Japanese missions to Tang China. Kan-on should not be confused with tō-on (唐音, Tang sound), which were later phonetic loans.

Kan-on are on'yomi (音読み) based on the central Chang'an pronunciation of Middle Chinese. The syllable Kan is a reading of Middle Chinese: 漢 (xan) as per Japanese phonology, referring to the Han dynasty, which had Chang'an as its capital city. Furthermore, Kan (漢) has also become a description for all things Chinese, e.g., kanji ('Chinese characters').