Definition of keystone noun from the Oxford Advanced Learner's Dictionary

keystone

noun
ˈkiːstəʊn
 
; ˈkiːstoʊn
 
 
1 (architecture) the central stone at the top of an arch that keeps all the other stones in position2 [usually singular] the most important part of a plan or argument that the other parts depend onChanges to the welfare system are the keystone of the government's reforms.