keystone
n. countablen. the most important part of a plan or idea that holds everything else together. In building, it is the middle stone at the top of an arch that keeps the other stones in place.
n. the central wedge-shaped stone at the summit of an arch, locking the others in position; figuratively, the central principle upon which a system or policy depends.
Trust is the keystone of any healthy relationship.
The architect explained that the keystone was essential for the structural integrity of the stone bridge.
The freedom of the press is often described as the keystone of a democratic society, without which the entire structure of accountability would collapse.
From key + stone. Definition 4 (retail) possibly originated in the jewelry industry in the magazine called "Jewelers' Circular-Keystone".
Often used metaphorically in political or academic contexts to identify a primary supporting element.