key +‎ point

Noun
keypoint (plural keypoints)

Part of a valley where the lower and flatter portion of the primary valley floor suddenly becomes steeper, used in computing a keyline for the efficient planning of water usage.

An important point in an image, used in computer vision systems when detecting objects etc.

keystone
/ˈkiːstəʊn/
noun
a central stone at the summit of an arch, locking the whole together.

Similar:
cornerstone
central stone
quoin
the central principle or part of a policy, system, etc., on which all else depends.