Anything that flows is called as fluid. In this sense liquids and gases are both considered as fluid. The study of properties of fluid at rest and fluids in motion is called Fluid Mechanics. This topic used to be studied in detail as a chapter and taught in our A Level Physics tuition classes, before the H1 H2 syllabus was launched some time back in 2007. Now the concepts of fluid mechanics are dealt with less robustly, in the form of Archimedes Principle in the chapter of Forces.
When we look around we find that we are surrounded by fluids. We have winds blowing across countries bringing different weathers, as well as the haze we are experiencing now! We have oceanic tides that follows the near sinusoidal motion. We also have important man-made examples of applications of fluid. All the pumps in the world that supply water to people is one such example. We have airplanes using fluid (air) to soar in air. We have fluids burning inside our vehicles that help us in transport. They are governed by the laws of fluid mechanics, such as those of Bernoulli’s principle in their motion.
Contrary to these examples of fluids in macro world, fluids are also seen in micro world. For instance, how the plants suck underground water for their nourishment from passages that are in micrometers to millimeters. Also, we are continually experiencing the flow of blood and other fluids inside our body. All in all, the world of fluid mechanics is fascinating and at the same time very complex, especially when we venture into the field of Chaos.