<p><strong>Noun</strong></p><ul><li>The wedge-shaped piece at the summit of an arch, regarded as holding the other pieces in place.</li><li>Something on which associated things depend: the keystone of one’s philosophy.</li><li>Synonyms: linchpin, foundation, principle, basis.</li><li>Also called keystone sack.</li><li>Baseball Slang: second base.</li></ul><p><strong>End of Response</strong></p>