An oni is a kind of yōkai, demon, orc, ogre, or troll in Japanese folklore. They are believed to live in caves or deep in the mountains. Oni are known for their superhuman strength and have been associated with powers like thunder and lightning, along with their evil nature manifesting in their propensity for murder and cannibalism. They are typically portrayed as hulking figures with one or more horns growing out of their heads, massive teeth, and occasionally a third eye in the center of the forehead. They are typically depicted with red, blue, black, or yellow colored skin, wearing loincloths of tiger pelt, and carrying iron kanabō clubs. They also have three to six digits on each hand and foot tipped with claw-like nails. Oni are able to change their looks to fool their victims into trusting them. Oni can be male or female, but have been predominantly male throughout history. Female oni are sometimes referred to by the name Yamauba. When in disguise, oni are capable of appearing as a man or woman, regardless of their gender. As monstrous as oni are, they have been linked to bringing good fortune and wealth.

Yōkai are a class of supernatural entities and spirits in Japanese folklore. The word yōkai is composed of two kanji characters that both mean "suspicious, doubtful", and while it may be regarded as a loanword from the Chinese term yaoguai, the word yōkai has taken on multiple different meanings particular to a Japanese context.