Critical, decisive; subject to significant or frequent changes. Cf. climacteric, adj. 1b.

Of a particular moment, period of time, etc.: of decisive importance with regard to the outcome of a situation.

That finally decides between two rival hypotheses, proving the one and disproving the other; more loosely, relating to, or adapted to lead to such…

Allowing no middle course between success and failure; crucial, critical. Cf. to make or break at make, v.1 69b.

Critical, decisive, crucial; involving or arising from a crisis. colloquial.