n.
Pronunciation: ' k ē - ˌ st ō n
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