The "
turn" towards the end of a
story that suddenly gives it not only a
happy ending, but a meaning for all that has gone before, however terrible the events before. The eucatastrophe is the point at which your heart skips and you start to cry tears of
joy.
Eu = 'good, well'; it's the good turning around.
The term was coined by J.R.R. Tolkien in his 1937 lecture On Fairy-Stories, in contrast with the catastrophe of tragedy. He regarded it as an especially salient part of the fairy-stories and other such great primal works of imagination: those that give Consolation. He says the "eucatastophic tale is the true form of fairy-tale, and its highest function."
Tolkien, as a Christian, regarded human sub-creation of worlds in stories as a reflection of God's creation of the Primary World, and the Incarnation as the eucatastrophe of this world's story.
He ended his essay quoting from a fairy tale, "The Black Bull of Norroway". The strongest example I've ever known of a eucatatrophe was in The Railway Children, and if you don't know it, you should, and if you do know it you know exactly what's coming: on the second-last page,
"Oh! my Daddy! my Daddy!'