Crux
Etymology
From Latin crux, “cross.” In English it first meant a torment or puzzle — a “cross” one had to bear, hence a knotty difficulty — and from “the crucial difficulty” (the same root gives us crucial and excruciating) it settled into “the central, decisive point.”
noun · /KRUKS/
The decisive or most important point of an issue.
What it names
The make-or-break heart of a problem — the one point on which everything else turns, as distinct from the surrounding detail.
Example
We argued about budgets for an hour, but the crux was whether we trusted the vendor at all.
Related idea
The crux is the single point that must be settled; the gist is the general substance or main drift of the whole.