Final answer:
A stream tube is a conceptual model for analyzing the flow of fluid through an area in fluid dynamics and aerodynamics, where the flow is consistent and no mass is exchanged with the surroundings.
Step-by-step explanation:
A stream tube is a conceptual model used in fluid dynamics and aerodynamics to analyze the flow of fluid through an area of varying size and shape. If we consider the flow of water in a river, the stream tube can be imagined as an invisible tube or channel within the river where the flow of water is consistent and no mass is lost or gained within that tube. The dimensions of the tube can change along the path of the flow, and this principle can be seen in action in various physical instances such as blood flowing through a blood vessel, air moving through a Venturi constriction, or water being pushed up through a drinking straw due to difference in air pressure.