The top stone of an arch.
"The mason carefully placed the heavy keystone at the apex to complete the grand archway."
Usage: In architecture, the keystone is the central wedge-shaped stone that locks an arch together and bears its weight. In photography or computer vision, "keystoning" refers specifically to correcting perspective distortion caused by angled projection surfaces.
To distort (an image) by projecting it onto a surface at an angle, which for example causes a square to look like a trapezoid.
"The photographer had to digitally un-keystone the building photo so that the vertical lines appeared perfectly straight instead of converging toward the top."
In plain English: To keystone means to be so important that if you are removed, everything else will fall apart.
"The keystone arch is considered an architectural masterpiece because its central stone locks all others in place, supporting the entire structure above it."
A census-designated place in Colorado.
"Residents of Keystone, Colorado, gather at the town square to watch the annual ski competition."
The word keystone is a compound of key and stone, referring to the central locking block in an arch that holds all other stones together. The specific retail meaning likely emerged from the name of the jewelry trade publication, Jewelers' Circular-Keystone.