What does 妖怪 Yokai mean?

Yokai is usually translated as monsters or spirits. The word is built from two characters that both mean something like strange, and what it names is the experience of the inexplicable rather than a kind of creature.

Two characters, one idea, said twice

妖 (you) covers the bewitching and the uncanny, with an edge of the calamitous. 怪 (kai) covers the mysterious and the suspicious, the sense of something being off. Japanese compounds them.

Why it is not a species

English organizes the supernatural by what a thing is. Ghosts are dead people. Monsters are creatures. Fairies are their own order. Each label answers a question about ontology.

Yokai does not work that way. It admits animals that have lived long enough to acquire power, household objects that reach a certain age, faceless figures met on a road, and phenomena with no body at all: a sound in an empty street, a weight that settles on a sleeper, a stretch of path that cannot be walked to its end. What these share is not a nature but an effect, which is that something has happened and it will not resolve.

The people who made it a subject

Kunio Yanagita (1875 to 1962) is generally called the founder of modern Japanese folklore studies,. His interest was in what the stories revealed about the places that told them, which turned local strangeness into a record of regional life.

Shigeru Mizuki did the other half. His long-running manga about a yokai boy, begun in the 1960s, is best known for popularizing yokai.

Where the English translation costs something

Rendering it as monsters keeps the creatures and discards everything else. The bodiless cases are the interesting ones, because they show what the category is for: it is a way of naming an experience that has no explanation, without first deciding whether the cause was a creature, a place or nothing at all.

English has no equivalent container. The nearest is the paranormal, which is clinical and modern and carries an implied argument about whether the thing is real. Yokai carries no such argument, which is why a household object can become one simply by lasting.