rheology (rēŏlˈəjē) [key], branch of physics dealing with the deformation and flow of matter. It is particularly concerned with the properties of matter that determine its behavior when a mechanical force is exerted on it. Rheology is distinguished from fluid dynamics (see fluid mechanics) in that it is concerned with all three of the traditional states of matter rather than only with liquids and gases. Unlike polymer physics it is concerned with macroscopic properties and behavior and not with molecular structure. The results of rheology provide a mathematical description of the viscoelastic behavior of matter (see elasticity; viscosity). Applications of rheology are important in many areas of industry, involving metals, plastics, and many other materials.