A fluid is a substance that can flow. A liquid is a fluid, but smoke and air can also be considered fluids. Fluids also have the tendency to assume the shape of the container housing them. There are many physical phenomena associated with fluids and there is an entire branch of physics concerned with studying fluids and the motions of such systems called fluid dynamics. Gases with a large number of highly charged particles (plasmas) are also considered fluids, and the study of plasmas is an important subcategory of fluid dynamics.

When a fluid is in motion it can move in several different ways. At one extreme the flow of the fluid can be laminar or streamlined; at the other, the motion can be turbulent. In a laminar flow every point that passes a particular point moves along the exact path followed by the particles that preceded it...