keystone
noun [ C ] uk/ˈkiː.stəʊn/ us/ˈkiː.stoʊn/
(STONE)
architecture specialized拱顶石;塞缝石
the middle stone in the top of an arch that has a special shape and holds all the other stones in position
(IMPORTANT PART)
（计划、想法等的）基础，主旨，基本原则
the most important part of a plan, idea, etc. on which everything else depends