Definition of "keystone"

keystone

/ˈkiː.stəʊn/

noun
plural keystones
1. (architecture) The top stone of an arch.
  Quotations
  The tunnel, which is 1,000 yd. long, had been closed to passenger traffic since November 17, 1949, after some of the keystones had worked loose.
  1950 March, “Notes and News: Sugar Loaf Tunnel, Central Wales Line”, in Railway Magazine, page 209
2. Something on which other things depend for support.
  Quotations
  Tension between empirical and theoretical knowledge is keystone to sociological and to organizational theories, as early as in Marx's and Weber's frameworks.
  1999, Eliezer Geisler, Methodology, Theory, and Knowledge in the Managerial and Organizational ...‎
3. A native or resident of the American state of Pennsylvania.
4. (retail) A retail price that is double the cost price; a markup of 100%.
5. (baseball) The combination of the shortstop and second baseman.
verb
third-person singular simple present keystones, present participle keystoning, simple past and past participle keystoned
1. (transitive) To distort (an image) by projecting it onto a surface at an angle, which for example causes a square to look like a trapezoid.
2. (transitive, retail) To double the cost price in order to determine the retail price; to apply a markup of 100%.