Abbrev. for input-process-output. A method for representing system designs in terms of system functions and relations between system functions. The method provides a simple diagram notation. Each IPO diagram has a name describing the function to be performed, usually some reference number, and a substructure of three rectangular boxes, one each for the input, process, and output for the named function. Relationships between IPO diagrams are shown as a functional hierarchy with unnamed links between boxes named and/or referenced.