Curvature is understood to be a deviation from the straight.
When an endless material is wound onto a spool, the inner layers have a smaller radius of curvature and the outer layers a larger radius of curvature.
Whether or not a winding is elastic or plastic depends on the spool geometry, the properties of the endless material, and the degree of deformation.
Curvature is the reciprocal of the radius of curvature and has the dimension [1/mm].
See Also
Adapted from “"We do it straight" — Wire Straightening”, p. 57 (ISBN 3-00-005897-4).