Matter is generally divided into three different phases, namely – solid, liquid and gas. A solid is made up of atoms which are held together rigidly by strong inter-atomic forces. Solids have definite shape and volume. The shear stress i.e., the tangential force applied to a surface per unit area, introduces deformation or change in shape in solid but without changing its volume as shown in the Fig.1.

             The liquid has a definite volume and it takes the shape of the container. However in gas, both shape and volume are same as that of its container. Both liquids and gases can not sustain any shearing stress. When a shear stress is applied on the top surface of a liquid (gas), a layer of liquid (gas) at the top surface flows away and the top surface is replaced by a fresh layer of liquid (gas).

              A fluid is a substance which can flow. A fluid cannot sustain shear stress as explained above for liquid and gas. So, liquids and gases can be called as fluids. The basic difference between liquid and gas is that liquid is incompressible and gas can be compressed easily.