ON-reading
The On Reading or On'yomi (音読み) is the Chinese translation for a kanji.
The Kun Reading, Kun'yomi (訓読み), or Native Reading is the Japanese translation for a "kanji" (Chinese characters). Since Japanese kanji originated in China a lot of them use the same meaning and sound, but some are different, or they use both readings depending on the word. There are even some Kanji that have over 30 readings depending on the word. Over all there are over 2000 of these kanji, which make up of one of the three writing systems of the Japanese language.
For more information, see KUNReading or the Wikipedia article on kanji
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