Liminal: Related to a threshold. Used especially in relation to a psychological state between existential planes (a liminal state between life and death). From Latin limin or limen, "threshold."
Liminal is also occasionally used in architecture to refer to a physical threshold. In physics, liminal refers to the least amount (of energy, for example) required to produce an effect.
A legal term, in limine, is linguistically related. A motion in limine ("at the threshold") is a request submitted to the court in an attempt to exclude evidence during trial, usually to shield the jury from possibly prejudicial evidence.
UPDATE: I wrote this post over the weekend. This morning my A Word a Day subscription brought me--yep, you guessed it, "liminal." Coincidence? Maybe we're all feeling as though we're on the threshold of something.