I have been struggling with this question since I wrote the ending to my novel in May 2010. I now have the answer, thanks to an excellent post by K.M. Weiland:
All the other possibilities I considered it turns out are actually the” inciting event,” the key event,” the “first plot point,” and something she so aptly calls “throat clearing.” I labeled it “Prologue.” All 54 pages… Turns out it’s back story, and may have to stay in my head.
No more excuses to procrastinate about editing in earnest. Thank you KM!