This chapter presents an ISD ontology, which aims to provide an integrated conceptualization of ISD through anchoring it upon a contextual approach. The ISD ontology is composed of concepts, relationships, and constraints referring to purposes, actors, actions, and objects of ISD. It is presented as a vocabulary with explicit definitions and in meta models in a UML-based ontology representation language. We believe that although not complete the ISD ontology can promote the achievement of a shared understanding of contextual aspects in ISD. It can be used to analyze and compare existing frameworks and meta models and as a groundwork for engineering new ISD methods, and parts thereof.