Definition from Wiktionary, the free dictionary
streamline (plural streamlines)
- (physics) A line that is tangent to the velocity of flow of a fluid; equivalent to the path of a specific particle in that flow.
- (meteorology) On a weather chart, a line that is tangent to the flow of the wind.
line that is tangent to the velocity of flow of a fluid
meteorology: line that is tangent to the velocity of flow of the wind
See also 
- (transitive) To design and construct the contours of a vehicle etc. so as to offer the least resistance to its flow through a fluid.
- (transitive, by extension) To simplify or organize a process in order to increase its efficiency.
- (transitive) To modernise.
Derived terms 
to design an object so as to offer least resistance to its flow through a fluid
to simplify a process