Fluid, a form of matter that has no definite shape. Gases and liquids are fluids, but solids are not. When a fluid is put into a container, it adapts itself to the shane of the container. When a quantity of liquid is put into a container, it takes on the shape of the container for as much of the volume as it occpies. A quantity of gas, on the other hand, not only adopts the shape of the container but diffuses, or spreads out, to fill all of the container. The same quantity of gas can also be compressed to fit into a smaller container by applying pressure. A liquid, however, changes its size very little even when subjected to considerable pressure.
The ease with which a fluid flows is determined by its viscosity: The more viscous the fluid, the less easily it flows. The branch of science that studies the properties of fluids is called fluid mechanics.
