/kee"stohn'/ , n.
1. the wedge-shaped piece at the summit of an arch, regarded as holding the other pieces in place.
2. something on which associated things depend: the keystone of one's philosophy.
3. Also called keystone sack . Baseball Slang. See second base (def. 1).
[ 1630-40; KEY 1 + STONE ]
Syn. 2. basis, principle, foundation, linchpin.