Fulcrum
Etymology
From Latin fulcrum, “bedpost, support,” from fulcire, “to prop up, support.” It named a physical prop, then the resting point of a lever in mechanics, and from there the decisive point on which events turn.
noun · /FULL-krum/
The fixed point on which a lever pivots — or, figuratively, the thing on which a situation turns.
What it names
The pivot that gives leverage — the small, well-placed support that lets a modest force move something far larger.
Example
One reluctant senator became the fulcrum of the whole negotiation; everything leveraged off her vote.
Related idea
A fulcrum is the pivot that provides leverage; a linchpin is the part whose removal makes the whole thing fall apart.