Abstract: This paper proposes an ontology of functional concepts of artifacts, which provides a rich and well-organized vocabulary for functional representation. It includes nine types of the roles played by a function for another function, named meta-functions, such as "to enable" and "to drive". They represent interdependence between functions and an important part of the design rationale of artifacts. It provides vocabulary for explanation to human designers and primitives specifying functional reasoning space for (re)design systems.