Final answer:
An IPO chart is a design tool used to describe the input, processing, and output of a function, which is critical in various fields like engineering design and economic modeling.
Step-by-step explanation:
The design tool that describes the input, processing, and output of a function is the IPO chart. This stands for Input, Processing, and Output. An IPO chart is a useful way to organize and represent the flow of data through a system. It helps clarify the process by dividing a complex function into its basic elements. Such a chart is paramount in many areas, particularly when discussing the engineering design process, or working with economic models which often can be represented graphically. Whether for designing a product and considering the budget for product development or understanding the relationship between two properties of a system, like the rate of the flow of water through a hose, graphical representations like the IPO chart provide a straightforward visualization of how systems function.