Plasticity
The degree to which a form reads as pliable or moldable rather than rigid, judged from its contours and transitions.
In practice
Plasticity describes how soft, moldable or flowing a form looks, judged by its contours and the way its surfaces pass into one another. Smooth, continuous curves read as plastic; sharp edges and flat faces read as rigid. A computer mouse with soft, rounded transitions looks as though a hand shaped it, while a boxy metal case does not. In lettering, rounded, swelling strokes create a similar feeling.
Often confused with plastic
Plasticity is a visual quality of soft, moldable-looking form; it has nothing to do with whether the object is actually made of plastic.
Try this
Model one shape twice in clay, once with crisp edges and once smoothly blended, and compare their character.