在机械工程学里，形变定义为由于外力作用而造成的形状改变，这外力可能是拉力、推力、剪力、弯力或扭力等。形变时常是用应变来描述。

如右图可见，压缩负载造成了圆筒的形变，原本的形状（虚线）已经改变（形变），圆筒的侧面凸出。圆筒虽然没有裂开或败坏，但其强度并不足以在负载下保持形状不变，因此侧面凸涨出来。

形变可能是暂时性的，就像放松的弹簧会回到原来的长度；形变也可能是永久性的，当物体不可逆地弯曲时便为永久形变。若过了一定的限度则不能恢复原状，这样的形变叫做塑性形变，此限度称作弹性限度。

In materials science, deformation refers to any changes in the shape or size of an object due to-

The first case can be a result of tensile (pulling) forces, compressive (pushing) forces, shear, bending or torsion (twisting).

In the second case, the most significant factor, which is determined by the temperature, is the mobility of the structural defects such as grain boundaries, point vacancies, line and screw dislocations, stacking faults and twins in both crystalline and non-crystalline solids. The movement or displacement of such mobile defects is thermally activated, and thus limited by the rate of atomic diffusion.