Rigid

Rigid is an adjective meaning not easily bent, twisted, or broken; lacking flexibility. The term describes objects that retain their shape under external forces and can also refer metaphorically to inflexibility in rules or attitudes. In science, rigid often denotes an idealization in which the distances between all points of an object remain constant regardless of external forces, an assumption that simplifies analysis.

In physics, a rigid body is an extended object whose internal distances do not change as it moves. Its motion is described by translations and rotations, ignoring any deformation. Real materials deform elastically or plastically, but the rigid-body approximation is useful when deformations are small or stiffness is high. In mathematics and geometry, a rigid transformation, or isometry, preserves distances and angles. Typical rigid motions include translations, rotations, and reflections; compositions of rigid motions are also rigid.

In engineering and materials science, rigidity relates to stiffness and is connected to elastic moduli such as Young's modulus, shear modulus, and bulk modulus, which quantify resistance to deformation. In computer graphics, rigid transformations map points to new positions without changing distances between points, in contrast to non-rigid or deformable transformations that allow scaling or bending.

In everyday language, rigidity also denotes inflexibility in behavior or policy, such as a rigid schedule or a rigid set of rules, where variation is discouraged or constrained.