what is a fluid

In physics, a fluid is a substance that can continuously move and deform under an applied shear stress or external force. This includes liquids, gases, and other materials that exhibit zero shear modulus and cannot resist shear forces applied to them. Fluids have properties such as lack of resistance to permanent deformation, and their behavior can be described by the Navier–Stokes equations, which are based on principles like conservation of mass, momentum, angular momentum, and energy. The study of fluids is known as fluid mechanics, which is further divided into fluid dynamics and fluid statics, depending on whether the fluid is in motion. In summary, a fluid is a state of matter that yields to sideways or shearing forces, and it includes liquids, gases, and materials that exhibit similar behavior