noun
ko-on; ancient reading; old Chinese-derived reading
A technical term in Japanese historical linguistics referring to a layer of Sino-Japanese readings based on early Chinese pronunciations (Zhou, Han, Wei dynasties). Contrasts with go-on, kan-on, and tō-on.
「古音」は、漢字の古い時代の音読みを指す専門用語です。
Ko-on is a technical term for the old Sino-Japanese readings of kanji.
Tō-on (or sō-on) refers to later borrowings from Song dynasty and later Chinese, distinct from the ancient ko-on layer.
Compound of 古 (old) and 音 (sound, reading). The term itself is a modern linguistic classification; the readings it describes are early borrowings from Chinese.