- the wedge-shaped piece at the summit of an arch, regarded as holding the other pieces in place.
- something on which associated things depend: the keystone of one’s philosophy.
- Also called keystone sack. Baseball Slang. second base(def 1).
- Also called: headstone, quoin the central stone at the top of an arch or the top stone of a dome or vault
- something that is necessary to connect or support a number of other related things
“stone in the middle of an arch, which holds up the others,” 1630s, from key (n.1) in figurative sense of “that which holds together other parts” + stone. Figurative sense is from 1640s. Pennsylvania was called the Keystone State because of its position (geographical and political) in the original American confederation, between northern states and southern ones. Keystone cops were the bumbling police in the slapstick silent movies produced by Keystone Company, formed by Canadian-born U.S. film director Mack Sennett (1884-1960) in 1912.