This paper proposes a failure-related definition of interdependencies. The concept of interdependencies has never been discussed with a level of formality that could support its systematic treatment. To be used in engineering settings, interdependencies cannot be just a generic term. No valid assessment of interdependencies can be made devoid of a clear constraint of the semantic value of the concept. The most cited description of interdependencies refers to "linkages or connections", but without any mention of failure conditions. This paper defends that interdependencies implies the occurrence of impairments with dependability implications. Therefore, in a risk-oriented security assessment of interdependencies, are only relevant the interrelationships among systems (structural interconnections, behavioural interactions, functional couplings) that are related to the creation and manifestation of faults, errors and failures.