One of the most cited definitions of interdependency is the one by Rinaldi et al. who define interedependency as a bidirectional relationship between two infrastructures through which the state of each infrastructure influences or is correlated to the state of the other 
Official European Definition
Mutually reliant relationship between entities (objects, individuals, or groups) . The degree of interdependency does not need to be equal in both directions. For example, the Energy Sector depends on a functioning Water Sector for successful day-to-day operations, just as the Water Sector needs energy to effectively manage water and wastewater systems .
- Steven M. Rinaldi, James P. Peerenboom, Terrence K. Kelly, Identifying, Understanding and Analysing Critical Infrastructure Interdependencies, IEEE Control Systems Magazine, December 2001, p.14.
- DHS Risk Lexicon 2010 Edition, September 2010
- Critical Infrastructure Protection and Resilience Toolkit (Glossary), US Department of Homeland Security
- Unpublished working glossary of UP KRITIS and BSI, 2014