Linchpin
Etymology
From Middle English lynspin: lyns (from Old English lynis, “axle-pin”) + pin. It is literally the pin pushed through the end of an axle to keep the wheel from rolling off — pull it and the wheel, and the whole wagon, comes apart, which drives the figurative force.
noun · /LINCH-pin/
The single element that holds an entire system or plan together.
What it names
The one part whose removal makes everything else fall apart, pinpointing critical dependence rather than mere importance.
Example
She turned out to be the linchpin of the project; when she left, three deadlines quietly collapsed.
Related idea
Both name a load-bearing part, but a linchpin secures a moving mechanism, while a keystone locks a static structure in place.
noun · /LINCH-pin/
A pin passed through the end of an axle to keep a wheel from sliding off.
What it names
the small pin that secures a cartwheel
Example
Lose the linchpin and the wheel rolls free.
Related idea
The figurative “thing that holds it all together” comes directly from this humble pin; compare keystone.