elastic deformation

Stretch a spring gently, let go, and it snaps back to exactly its old length. That fully recoverable, springs-back deformation is elastic deformation. At the atomic level nothing has permanently rearranged: the bonds between atoms have merely been stretched or bent a little, like tiny stiff springs, and they pull the atoms straight back when the force is removed.

In elastic deformation stress and strain are proportional (Hooke's law), so the stress-strain curve starts as a straight line whose slope is the elastic modulus. Because it is only bond-stretching, the strains are small — for stiff metals typically under about 0.5 percent before yielding. Take a load off within this range and the strain returns to zero with no memory of having been loaded. Elastic deformation also stores energy: that energy, released on unloading, is what makes a spring push back or a bow launch an arrow.

The key honest point is that elastic behaviour is bounded. Push past the elastic limit and some deformation becomes permanent (plastic), and the bar no longer returns fully to its start. Elasticity is also usually reversible only in shape, not in every detail: real materials show tiny time-lag effects (viscoelasticity, more so in polymers), and 'elastic' does not mean 'stretchy' — glass and diamond are extremely elastic in the technical sense yet barely deform before they break.