Definition of "peripeteia"

Once more I was a man in a myth, incapable of understanding it, but somehow aware that understanding it meant it must continue, however sinister its peripeteia.

1965, John Fowles, The Magus

They were to bestride the Algerian scene like demigods until the tragic peripeteia of 1961 […]

1977, Alistair Horne, A Savage War of Peace, New York Review books, published 2006, page 167

The visual moment whose consequences Freud began to ponder in the essay on the phallic stage has evolved into a peripeteia: "Some day or other it happens that the child whose own penis is such a proud possession obtains a sight of the genital parts of a little girl; he must then become convinced of the absence of a penis in a creature so like himeself. With this, however, the loss of his own penis becomes imaginable, and the threat of castration achieves its delayed effect."

1989, Elizabeth Abel, Virginia Woolf and the Fictions of Psychoanalysis, page 6