One of the most cited definitions of interdependency is the one by Rinaldi et al. who define interdependency as a
This definition is adopted on the Commission's staff working document on a new approach to the EPCIP :
The working document also highlights that threats to a single Critical Infrastructure can have "a very significant impact on a broad range of actors in different infrastructures and more widely". The document also highlight the cross-border effect of interdependencies, spanning a number of European countries. The example identified in the document is the European high-voltage electricity grid, composed of the interconnected national high-voltage electricity grids.
European Project Definitions
The CIPRNet project  uses the following definition:
An older definition was:
The degree of interdependence does not need to be equal in both directions.
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 .
Empirical analysis of Critical infrastructure incidents and events by Van Eeten et al. shows that interdependencies (= the mutual Dependency) hardly occur; and when they occur the effect is at a lower (weaker) dependency level.
- Steven M. Rinaldi, James P. Peerenboom, Terrence K. Kelly, Identifying, Understanding and Analysing Critical Infrastructure Interdependencies, IEEE Control Systems Magazine, December 2001, p.14.
- Staff Working Document on a new approach to the European Programme for Critical Infrastructure Protection - Making European Critical Infrastructures more secure, SWD(2013) 318 final, Brussels, 28.8.2013
- Glossary of the Government of Queensland
- GUIA DE REFERÊNCIA PARA A SEGURANÇA DAS INFRAESTRUTURAS CRÍTICAS DA INFORMAÇÃO Versão 01 (Nov. 2010).
- Protection of Critical Infrastructures – Baseline Protection Concept: Recommendation for Companies, BMI.
- UP KRITIS, BSI, 2014
- PROTEZIONE DELLE INFRASTRUTTURE CRITICHE INFORMATIZZATE La realtà Italiana (2004)
- Comprehensive Disaster Management Policy Framework for Trinidad and Tobago
- [http://www.cnpic.es/Biblioteca/Legislacion/Generico/Ley_8-2011_PIC.pdf 7630 Ley 8/2011, de 28 de abril, por la que se establecen medidas para la protección de las infraestructuras críticas.
- DHS Risk Lexicon 2010 Edition, September 2010
- Critical Infrastructure Protection and Resilience Toolkit (Glossary), US Department of Homeland Security