Software and systems improvement requests to merge various interpretations from several improvement models and techniques. A particular challenge is the multitude of models for requirements and quality, which can get time consuming and error prone to trace, change, and verify. Lately, Ontologies have been used across several domains and for numerous purposes to be applied for many applications. Besides, recent work in Artificial Intelligence is discovering the use of formal ontologies as a way of identifying content-specific agreements for the sharing and reuse of knowledge among software entities. Therefore, this paper describes how ontology engineering is used to construct an Ontological structure of the proposed SPI-CMMI framework –which based on using Six sigma approach integrated with CMMI-Dev model and Quality Function Deployment (QFD) technique- with its progressive phases, related activities, recommended tools and the CMMI-Dev 1.3 representation. The SPI-CMMI Ontology provides a shared improvement terminology, defines precise and unambiguous semantics for the software enterprises and enables reuse of improvement phase's knowledge; in addition it makes domain assumptions explicit and separate domain knowledge from the operational knowledge.