Main Entry: key·stone

Pronunciation: \ˈkē-ˌstōn\

Function: noun

Date: circa 1637

1 : the wedge-shaped piece at the crown of an arch that locks the other pieces in place — see arch illustration
2 : something on which associated things depend for support <determination, a keystone of the puritan ethic — L. S. Lewis>
3 : a species of plant or animal that produces a major impact (as by predation) on its ecosystem and is considered essential to maintaining optimum ecosystem function or structure