keystone

[ kee-stohn ]
/ ˈkiˌstoʊn /

noun

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).

Origin of keystone

First recorded in 1630–40; key1 + stone
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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British Dictionary definitions for keystone

keystone

/ (ˈkiːˌstəʊn) /

noun

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
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012