keystone

Short definition from Wiktionary

Noun

1. 1
   The top stone of an arch.
2. 2
   Something on which other things depend for support.
3. 3
   A native or resident of the American state of Pennsylvania.
4. 4
   A retail price that is double the cost price; a markup of 100%.
5. 5
   The combination of the shortstop and second baseman.

Verb

1. 1
   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. 2
   To double the cost price in order to determine the retail price; to apply a markup of 100%.

Proper noun

1. 1
   A census-designated place in Colorado.
2. 2
   A city and town in Iowa.
3. 3
   A ghost town in North Dakota.
4. 4
   A town in South Dakota.