keystone - definition and synonyms

noun [countable] [usually singular] 

Your browser doesn’t support HTML5 audio

/ˈkiːˌstəʊn/
singularkeystone
pluralkeystones
  1. 1
    something that is very important to a plan, arrangement, or set of beliefs
  2. 2
    a stone at the top of an arch that keeps the structure together