Technology

Plastic and elastic deformation are both terms used to refer to the behavior of a material under stress (such as compression, tension, or torsion).

For an ELI5 since I don't feel like cracking open a material science textbook or really getting more nuanced than this for a basic explanation, elastic deformation is generally reversible without permanent changes to the structure of the material, while plastic deformation imparts a permanent change.

All materials have elastic and plastic deformation modes that can be identified based on their characteristic stress-strain curve. Generally, the linear portion of the curve at lower stresses is the elastic region, and the plastic region begins where the curve becomes nonlinear.

For example, a wooden beam in a house will bend under normal load. As people move out of the room that beam is in, it will straighten back out- that is elastic deformation. Put too many people or some very heavy furniture in the room, though, and the beam will become permanently bent or even break altogether- that is a plastic deformation.

Some solid books on this topic are Shigley's Mechanical Engineering Design and Roark's Formulas for Stress and Strain

The colloquial use of elastic and plastic to describe certain groups of materials is based off the behaviors of these modes of deformation. E.g. elastics are stretchy and return to their original shape. If you really want to get into semantics, there are only four types of materials: metals, polymers, ceramics, and composites. Everything else is one of those 4 things.