A fluid is a substance that deforms continuously under applied shear stress, regardless of size. Liquids and gases are collectively referred to as fluids. It cannot resist any shear force applied to it. This continuous deformation under the action of forces compels the fluid to flow. Fluids such as gases and liquids in motion are generally referred to as flow. Flow of fluid describes its behaviour and how they interact with their surrounding environment. Liquids are incompressible fluid substances whose density does not change with variations in temperature and pressure. Whereas, gases are compressible fluid substances whose density changes rapidly with changes in temperature and pressure. In the context of fluids, real fluids have viscosity, while ideal fluids are incompressible fluids without viscosity. A steady or unsteady flow is determined by the velocity of the fluid, which remains constant or differs over time. Smooth or regular movement of particles in the fluid is called laminar flow or viscous flow. In laminar flow, the velocity, pressure and other flow properties at each point in the fluid remain constant. Flow of a fluid that travel on an irregular path is called turbulent flow. In this flow, the speed of the fluid at a point undergoes changes continuously in both magnitude and direction.