‘Streamline’ is a term that originally came from science and technology. It refers to the straightest possible line along which a liquid or gas will flow. If the liquid encounters an obstacle, resistance occurs and the straight flow is interrupted.
Resistance or ‘drag’ slows vehicles down, can make them harder to control and increases their fuel consumption. So it is important that they are given the right shape to enable them to move efficiently through the air or water. This is how the streamlined design came about: designers use shapes that are often smooth and curved to guide the flow in the right direction.
The scientific study of the way air and other gases move is called aerodynamics. In the case of water or other liquids, it’s hydrodynamics.