A fluid is any material that can flow and cannot resist deformation. Fluids include gases, liquids, and plasmas. Some substances, such as Silly Putty, blur the distinction between solids and fluids, depending on the timescale over which they are observed.
There are three ordinary states of matter – solid, liquid, and gas – plus plasma, which can be considered a fourth state. The substance H2O is commonly called "ice" in its solid state, "water" in its liquid state, and "water vapor" in its gaseous state. When side forces, called shearing forces, are applied to a solid piece of ice, very large forces are needed to deform or break it. The solid has a very high internal friction, or resistance to shearing. The word for internal friction is viscosity and for a solid, its value is generally very large. Liquids have a lower viscosity than solids, and gases have a still lower viscosity, in other words, they have less internal friction.
Liquids and gases are both considered fluids since they behave differently from solids. Imagine two layers of water or air. If shear forces are applied to these layers, there will be a substantial and sustained relative motion of the layers with the air layers sliding faster over one another than the water layers. However, the fact that a shear force must be applied to deform both of these fluids indicates that they also possess internal friction.
Water, under normal temperatures, is about 50 times more viscous than air. Ice is 5 × 1016 times more viscous than air. One concludes that, in general, solids have extremely high viscosities whereas fluids have low viscosities. In the category of fluids, liquids generally possess higher viscosities than gases. Air, of primary interest in aerodynamics, has a relatively small viscosity, and in some theories, it is described as a perfect fluid – one that has zero viscosity or is "inviscid." But even this small viscosity of air (or internal friction) has important effects on an airplane in terms of lift and drag.
All fluids are compressible (that is, their density increases under increasing pressure) to some extent, but liquids are much less compressible than gases and are generally considered incompressible. Even gases may be treated as incompressible provided the airflow speeds involved are not great. For subsonic airflow over an airplane below about 150 m/s (492 ft./s or about 336 mph), air may be treated as incompressible, i.e., the density remains the same throughout the flow. At higher speeds, the effects of compressibility must be taken into account.
See also fluid flow.