Which Tolkien essay introduced the term “eucatastrophe” for a sudden joyful turn in a story?

Tolkien introduced the term “eucatastrophe” in his essay On Fairy-Stories.

He used the word to describe the sudden turn from apparent disaster to joy or deliverance at the end of a fairy-story. Tolkien presented eucatastrophe as the opposite of catastrophe in the sense of narrative ruin: it is not a denial of danger, but a meaningful reversal after genuine peril.

On Fairy-Stories developed from a lecture Tolkien gave at the University of St Andrews in 1938. The essay was later published in the 1947 collection Essays Presented to Charles Williams and became one of Tolkien’s most influential statements about fantasy. He connected the idea with what he called the “Catholic” or “Christian” joy of the story, while also treating it as a broad feature of successful fairy-tales rather than a device limited to one religion.