a simple geometric form capable of being flattened without stretching

a special ruled surface, which can be rolled out flat onto a plane without stretching of any portion Fitted Surface

a surface that can be flattened to form a plane without compressing or stretching any part of it. Examples include cones and cylinders.

In mathematics, a developable surface is a surface with zero Gaussian curvature. That is, it is surface that can be flattened onto a plane without distortion (i.e. stretching, compressing, tearing). Inversely, it is a surface that can be made by transforming a plane (i.e. folding, bending, rolling, cutting, and gluing).