Fluids can be defined as the substance that flows under the application of force or due to gravity it tends to flow. Liquids and gases both are considered as fluids. Plasmas are also considered as fluid. For example- brake fluid or hydraulic oil. It has no fixed shape as it deforms in the shape of the container it gets stored.
In fluid mechanics, liquids and gases are assumed to be incompressible that obey Bernoulli’s Theorem. Fluids are used in the different fluid systems to convert one form of energy into another form. Some of the fluid systems include torque converter, liquid volume converter, etc.