To construct or design in a form that offers the least resistance to fluid flow: streamline a car's design. To design with flowing, graceful lines: streamline furniture. To improve the efficiency of, often by simplification: streamline a factory process. A line that is parallel to the direction of flow of a fluid at a given instant. The path of one particle in a flowing fluid. A contour of a body constructed so as to offer minimum resistance to a fluid flow.