key·stone 
/ˈki:ˌstoʊn/ noun
plural key·stones /ˈki:ˌstoʊn/ noun [count] 1 : a large stone at the top of an arch that locks the other stones in place 2 : something on which other things depend for support
▪ Tourism is the city's economic keystone. ▪ the keystone of his faith
▪ Tourism is the city's economic keystone. ▪ the keystone of his faith
