cor·ner·stone / ˈkôrnərˌstōn/ • n. a stone that forms the base of a corner of a building, joining two walls. ∎ a stone ceremonially laid usually at the corner of a foundation to mark the occasion of a building being erected. ∎  an important quality or feature on which a particular thing depends or is based.

cornerstone a stone that forms the base of a corner of a building, joining two walls; an important quality or feature on which a particular thing depends or is based (often in biblical allusions, as in Isaiah 28:16).