ˈ ̷ ̷ ̷ ̷ˌ ̷ ̷ noun
Etymology: Middle English cornerston, from corner + ston stone
1. : a stone forming a part of a corner or angle in a wall and especially lying at the foundation of a principal angle ; specifically : such a stone laid at the formal inauguration of the erection of a building, usually inscribed with the date or other matters, and often hollowed out to receive records, documents, or relics
2. : the event, fact, or thing that forms the principal foundation or support upon which an achievement is based or from which a development makes its beginning
this first bill is the cornerstone of the administration's economic policy
: the principal or fundamental element, feature, or part of something
natural selection remains the cornerstone of evolutionary theory — E.W.Sinnott