What is the meaning of Keystone?

The top stone of an arch.

Something on which other things depend for support.

A native or resident of the American state of Pennsylvania.

A retail price that is double the cost price; a markup of 100%.

The combination of the shortstop and second baseman.

To distort (an image) by projecting it onto a surface at an angle, which for example causes a square to look like a trapezoid.

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

Source: wiktionary.org