The aim of this paper is to identify the key characteristics and interconnections of Critical Infrastructure and present a common definition that is informed by national and academic sources. A fundamental understanding of the interconnectivity and interdependencies that exist within Critical Infrastructure is essential to ensure global protection. This change in basic assumptions will require all nations to work together in the future to share knowledge and resources. The paper determines, through analysis, the foundation of Critical Infrastructure and its complexity given national and global links to support a global definition. The paper highlights the potential harm that can be caused if Critical Infrastructure links are not understood. A common definition for Critical Infrastructure is proposed and future work suggested.