Rigide

Rigide is an adjective used in French and Italian (and other Romance languages) meaning stiff or inflexible. In English, the corresponding term is rigid. The etymology traces to Latin rigidus, meaning stiff or hard. The concept appears across science, engineering, and everyday language to describe resistance to deformation or bending.

In physics and engineering, rigidity denotes the degree to which an object resists deformation under load. An ideal rigid body is a body that does not change shape or size when forces act on it; real materials deform according to elasticity, described by moduli such as Young's modulus and shear modulus. Rigidity is closely related to, but not identical with, stiffness, strength, and toughness. In structural analysis, the rigidity of a framework is captured by stiffness matrices and is essential for ensuring safe load transmission.

In geometry and graph theory, rigidity concerns whether a configuration is determined up to congruence by pairwise distances. A framework consisting of joints and bars is rigid if, under prescribed edge-length constraints, the only motions are rigid motions of space. In two dimensions, combinatorial results such as Laman's theorem provide criteria for minimal rigidity; higher dimensions involve more complex conditions, including global rigidity and redundant rigidity.

In everyday language, calling a person or process rigid communicates inflexibility, doctrinaire thinking, or strict adherence to rules. The term is also used in design and engineering to describe components or assemblies that maintain shape under expected service loads.