Definitionsare of the utmost importance in creating clarity about what is being talked about or what is being dealt with. A definition provides an unambiguous description of the subject, leaving no room for interpretation. However, when this freedom of interpretation is provided, it can lead to different perceptions of the subject, with all the consequences that entails. Looking at the field of Enterprise Architecture, it is important to have an unambiguous definition. In this chapter, the many definitions of Enterprise Architecture are presented. It is also pointed out that some essential parts are missing.