If there is a very important and irrecoverable moment which you have to make a vital decision, what do you call that moment in a single word?
There is "watershed moment" or "watershed decision", which means a turning point, after which everything (or lots of significant things) changed.
I'm afraid that I don't know the origin of this phrase (and have actually wondered about it before).
There is also "dilemma", which is less dramatic (and more common).
You may refer to it as a turning point:
- The point at which a very significant change occurs; a decisive moment.
You can also try critical juncture defined by Wikipedia in the article path dependence as
In the critical juncture, antecedent conditions allowcontingent choices that set a specific trajectory of institutional development and consolidation that is difficult to reverse.
Also see juncture at vocabulary.com where it says that the word has broadened in meaning to refer to an often urgent decision-making crossroads.
You may call that a point of no return.
n 1. a point at which an irreversible commitment must be made to an action, progression, etc - (Free Dictionary)