A rounded, bent, or arched shape in something is a curvature. When there's a wide expanse of open ocean, you can see the slight curvature of the earth right where it meets the horizon.
A curvature is the shape of something that's curved — and both words stem from the Latin curvare, "to bend." In math, the term curvature has several specific meanings depending on its context, but it basically comes down to a curving or bending away from a straight line. The curvature of your movie theater seat might make it particularly comfortable, while the curvature of your spine as you work on your computer may give you a backache later.