rigiditas

Rigiditas is a Latin noun meaning stiffness or resistance to deformation, derived from rigidus, “stiff.” In modern usage the term appears in scientific and historical contexts to denote stiffness across materials, structures, and biological tissues, often translated as “rigidity” in English.

In physics and engineering, rigidity describes an object’s resistance to shape change under applied forces. A rigid body is an idealization in which distances between points remain constant during motion. Rigidity is quantified by material properties such as Young’s modulus, shear modulus, and bulk modulus, as well as geometric factors like cross-section and moment of inertia that influence resistance to bending and torsion.

In geometry and topology, rigidity refers to properties that prevent a shape from being deformed without changing certain measurements. A framework is rigid if its shape is fixed up to rigid motions. The subject includes infinitesimal rigidity, global rigidity, and notable results such as Mostow rigidity, which constrain how geometric structures can be uniquely determined by certain data.

In materials science and engineering, rigidity concerns the stiffness of materials and structures under load. Temperature, microstructure, and loading rate affect rigidity. Engineers use modulus values to predict deflection and vibration, and distinctions are drawn between rigid, elastic, and plastic responses in design.

In biology and medicine, cells and tissues exhibit mechanical rigidity or stiffness, influencing processes like migration and differentiation. Abnormal rigidity occurs in fibrosis and cardiovascular disease, and neuromuscular rigidity is a clinical feature in conditions such as Parkinson’s disease.

See also rigidity, stiffnes.