日本語 Japanese Ask Questions Thread 8

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英語でも日本語でもOK
日本の方からのご協力も大歓迎です

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I have a very basic questions about kanji kun/on readings. For example, 真 has シン as onyomi. However http://jisho.org/words?jap=真 shows there's a word しん with meaning "truth, reality...". So how does that work? Does the onyomi シン have a specific meaning and is only (mostly?) used with that specific meaning in mind (I thought onyomi were meaningless)? Or is しん somehow an entirely separate word that happens to read the same?