Keystone
Etymology
From key, in the old sense of “that which fastens or holds together,” + stone. It is the wedge-shaped stone at the crown of an arch that locks all the others in place; remove it and the arch collapses. The architectural fact drives every figurative use.
noun · /KEE-stohn/
The central, locking element on which the stability of a whole structure depends.
What it names
The piece that is not merely important but load-bearing in a particular way — under pressure from every side, and holding precisely because of it.
Example
Trust was the keystone of the whole partnership; once it cracked, the rest gave way fast.
Related idea
A keystone holds a static structure together under compression; a linchpin holds a moving mechanism together.
noun · /KEE-stohn/
The wedge-shaped stone at the crown of an arch that locks all the others in place.
What it names
the central top stone of a masonry arch
Example
Remove the keystone and the arch collapses.
Related idea
In ecology a “keystone species” holds an ecosystem together the same way; compare linchpin.