There are two different types of readings for kanji which are 音読み/おんよみ/On-yomi and 訓読み/くんよみ/Kun-yomi. This article will briefly explain how these two came from.
The “On yomi” is based on the way it was read in China, although pronunciation is altered to the Japanese way. Some Kanji have more than one on-yomi reading. On-yomi was introduced to Japan via different routes. The earliest ones were introduced in Japan around the 5th and 6th centuries via the Korean Peninsula. The On-yomi reading was also imported by the Japanese envoys to the Tang dynasty and by monks who studied abroad in China in Nara and Heian periods (in the 700s). In addition, the arrival of books related to religion and active trade in the Kamakura period (1185 to 1333) imported On-yomi as well.
In dictionaries, the on-yomi is often displayed in katakana to distinguish it from the kun-yomi displayed in hiragana and to clarify that it is a phonetic transcription of a foreign word. However, in everyday documents, there is often no need to do so, and hiragana is often used.
The meanings of kanji may differ depending on how it is read. For example, kanji 悪 (meaning evil), is read あく/aku, then it means bad. But if it’s read お/o, the meaning changes to hate. Sometimes, Kanji was originally a heteronym in Chinese but has only one on-yomi in Japanese and is no longer a heteronym.
Kanji 訓 means to gently explain or paraphrase the meaning of a kanji character. The meanings of kanji characters were associated with the Japanese language that already existed. This is because Kanji characters were originally used to write Chinese which is a foreign language and do not match the meaning of Japanese words one by one. By the end of the Heian era (12th century), the Kanji-Japanese dictionary was made but some Kanji characters had more than 30 meanings in one character. This was too inconvenient and complicated, and so the rules were gradually changed to the form of “one meaning per one kanji” by the Muromachi period. With the establishment of Kun-yomi as a fixed Japanese reading for Chinese characters, there were no longer any difficulties in writing Japanese using Chinese characters.
Types of Kun yomi
Kun-yomi is applied to each word formed by several characters rather than each character. Because of this, when the word is broken down into single characters, the kun-yomi does not apply to each character.
The term refers to the application of kun-yomi according to the context rather than a fixed reading to a kanji. Rather than kun-yomi, it can be considered as a free translation of Chinese words into Japanese. The righteous Gikun may later come to be used in a fixed way and become the correct reading.
寒(cold) is read as ふゆ(Fuyu/winter)
暖(warm) is read as はる(Haru/spring)
This is when a Chinese character is used to express a Japanese original meaning. When this type of reading is used, the meaning of kanji largely differs from its original meaning in Chinese.
鮎/あゆ/Ayu/Freshwater trout - originally means catfish
鮭/さけ/Sake/Salmon - originally means puffer fish
芝/しば/Shiba/Grass - originally means mushroom
From foreign language
Kun-yomi used for kanji are not always Japanese words, but also can be foreign words. In this case, it’s usually written in Katakana.
頁/ぺーじ/Page - from English: Page
米/めーとる/Meter - from French: Mètre
麦酒/びーる/Beer - from Dutch: Bier
硝子/がらす/Grass - from Portuguese: Glas
To summarise, Kanji characters in Japanese have two readings, called "kun-yomi" and "on-yomi". The "on'yomi" reading is based on the way it was read in China. On the other hand, the kun'yomi is a reading style unique to Japan.