on'yomi & kun'yomi

("readings" of kanji). The on'yomi, the Sino-Japanese reading, is a Japanese approximation of the Chinese pronunciation of the character at the time it was introduced. Some kanji were multiply introduced from different parts of China at different times and so have multiple on'yomi as well as multiple meanings. On the other hand, the kun'yomi, the Japanese reading or native reading, is based on the pronunciation of a native Japanese word, or yamatokotoba, that closely approximated the meaning of the Chinese character when it was introduced. Again, there can be multiple kun readings for the same kanji, and some kanji have no kun'yomi at all. The rule of thumb for determining the pronunciation of a particular kanji in a given context is that kanji occurring in compounds are generally read using on'yomi. Kanji occurring in isolation -- that is, written adjacent only to kana, not to other kanji -- are typically read using their kun'yomi. (from Wikipedia).