What is the name of J.R.R. Tolkien's 1939 lecture that introduced the concept of eucatastrophe?

J.R.R. Tolkien introduced the concept of eucatastrophe in his 1939 lecture and essay On Fairy-Stories.

Tolkien delivered the Andrew Lang Lecture at the University of St Andrews in 1938, and the expanded text was published in 1939. In it, he discussed the nature and purpose of fairy stories, including fantasy, recovery, escape, consolation, and the “eucatastrophic” turn.

Tolkien used eucatastrophe for a sudden joyful change of fortune, especially one that brings hope after apparent disaster. He distinguished it from a simplistic happy ending: the effect depends on real danger and loss coming before the reversal. The essay is often read alongside Tolkien’s fiction because it provides his own critical vocabulary for understanding the emotional power of fantasy.