Enterprise engineering (EE) is a growing scientific discipline that takes the increasingly needed construction-oriented point of view, and aims to provide synthetic knowledge for letting enterprises more effectively and continuously adapt to threats and challenges. In this paper we present the τ-theory, being developed to support EE and consisting in the clarification of a series of foundational concepts of the EE domain like: system, model, subject, object, function, purpose and value, among others. Such clarification is then systematized in an ontology, together with an instantiation to validate it.