An event in history occured that changed the fundamental way humans live their day-to-day lives or the way humans see the world advanced to a point where it impacted the way all humans continue to exist.

Apocalyptic doesn't mean end-of-the-world necessarily. A more accurate way of putting it is that something huge was revealed. Kinda like how Mata Nui being a giant robot and his true purpose led to impacting the lives of all MU inhabitants and Agori alike. That's how I view it anyway.

I think of it as any event that wipes out/devastates the majority of humankind, whether instantly or over a period of time. In some cases humankind bounces back, in some cases it doesn't.