A viscous fluid is one which resists movement or the movement of an object through the fluid. All fluids, liquid, gas, or plasma, have some measure of viscosity which can be compared using mathematical formulas or direct measurements of movement. Though all fluids have viscosity, a viscous fluid, in the everyday sense of the term, is one that has a high level of viscosity. These types of fluid may move slowly or not at all, depending on how viscous they are.
In general, liquids measure between 1 and 1000 millipascal seconds, which is a common measure of viscosity. Gasses have much smaller viscosity measurements between 0.001 and 0.01 millipascal seconds. At sea level and room temperature, water has a measurement of about 1 millipascal second. This measurement is one of pressure, tensile strength, and movement and indicates the extent to which a fluid resists movement. A more viscous fluid will have a higher value in terms of millipascal seconds, while a less viscous fluid will have a smaller value.
The type of matter a fluid is made of is the main determiner of how viscous it is, though other factors, including temperature, will also affect viscosity. In general, liquids will become less viscous as their temperature rises, while gases will become more viscous with an increase in temperature. Gases become more viscous when they are heated because the atoms in the gas move more rapidly as temperature rises, resulting in more collisions between atoms and thus more resistance. Pressure also can affect viscosity, though this is not generally seen in liquids because, unlike gaseous matter, liquid matter is very difficult to compress.
A substance that would be referred to as a viscous fluid resists movement to some extent. This means that the fluid does not flow, or flows very slowly when a force, such as gravity is applied to it. It also means that it resists the movement of an object through it.
An extremely viscous fluid may have properties that make it behave more like a solid than a liquid. Butter is an example of a fluid with a high viscosity. Though butter does flow at room temperature, it is so resistant to movement that it is difficult to perceive it as a fluid. Heating butter will cause it to become noticeably less viscous. Glass is also a liquid. When glass cools and hardens into a solid-like state, its viscosity approaches infinity, meaning that it no longer flows at all.