<<event barrier..

*those square brackets were to show I didn't know what the English term was, and I was approximating it. The term, such as it is, pertains to the conditions under which a phenomenon can occur--under which, in this case, a black hole could come into being. Before Hawkings, it had been thought that a black hole could only come into being as the result of a cataclysmic event: one involving immense mass and energy. The article, as I remember, used the word "horizon," but the more familiar near-equivalent in English would be "barrier," as in "sound barrier," i.e., "the X-15 was the first manned vehicle to break the sound barrier." I put the term in brackets and gave both "horizon" and "barrier" because I hadn't solved the problem.

post edit

"threshhold" together with some modifier, I suppose, because "barrier" connotes an engineering challenge whereas "horizon" may suggest conditional boundaries but does not generally share that connotation (or does it.)

The challenge is out: find the term if it exists or invent it if it doesn't