Here’s another definition of “Threshold”. It is from a beautiful podcast from onbeing.org - an interview from 2008 with author, poet, theologian, and philosopher - John O’Donohue.

Well, I think that the threshold — if you go back to the etymology of the word “threshold,” it comes from “threshing,” which is to separate the grain from the husk. So the threshold, in a way, is a place where you move into more critical and challenging and worthy fullness. And I think there are huge thresholds in every life. I mean I think, for instance, to give a very simple example of it is that if you are in the middle of your life in a busy evening, 50 things to do, and you get a phone call that somebody that you love is suddenly dying — it takes ten seconds to communicate that information, but when you put the phone down, you are already standing in a different world, because suddenly, everything that seemed so important before is all gone, and now you are thinking of this. So the given world that we think is there, and the solid ground we are on, is so tentative.
And I think a threshold is a line which separates two territories of spirit, and I think that very often how we cross is the key thing.
Where beauty is, I think, is — beauty isn’t all about just nice loveliness, like. Beauty is about more rounded, substantial becoming. And I think, when we cross a new threshold, that if we cross worthily, what we do is we heal the patterns of repetition that were in us that had us caught somewhere. And in our crossing, then, we cross onto new ground, where we just don’t repeat what we’ve been through in the last place we were. So I think beauty, in that sense, is about an emerging fullness, a greater sense of grace and elegance, a deeper sense of depth, and also a kind of homecoming for the enriched memory of your unfolding life.
- John O’Donohue