on'yomi 音読み

In Japanese, on'yomi 音読み, also transliterated as onyomi, and sometimes written in dictionaries as just on 音, refers to the reading of a kanji 漢字 based on its original Chinese reading from the time the kanji were imported from China into Japan.

However, note that, since it's been quite some time since it happened, the current, modern Chinese pronunciation of the kanji is not the same as the Japanese pronunciation of the on 音 readings.

The counterpart of on'yomi is the kun'yomi 訓読み. Their differences were explained in kun'yomi vs. on'yomi.