Existential. If something is existential, it has to do with human existence.
I first heard the word associated with climate change and how the issue of a warming Earth is an "existential threat" to our existence as humans. It pops up on a regular basis in climate discussions and papers.
More recently, I'm starting to hear it used in other contexts, which (not recalling the specifics) has more to do with politics than human existence. Usually the word is used to say something like, "This action/person is an existential threat to ___________ (you fill in the blank). Politically, in today's environment, the threat might be to the "United States Constitution."
I've already tagged "unprecedented" as the 2019 word of the year, but coming in a distant second, maybe "existential."