An oni (鬼おに) is a kind of yōkai, demon.An old etymology for "oni" is that the word derives from on, the on'yomi reading of a character (隠) meaning "to hide or conceal", due to oni having the tendency of "hiding behind things, not wishing to appear". This explanation is found in the 10th century dictionary Wamyōshō, which reveals that the oni at the time had a different meaning, defined as "a soul/spirit of the dead"