Information and Communication Technology (ICT) is increasingly utilised in the electrical power transmission system. For the power system, ICT brings a lot of benefits, but it also introduces new types of vulnerabilities and threats. Currently the interdependencies between the power and ICT system are not fully understood, including how threats (both malicious and accidental) towards the ICT system may impact on power delivery. This paper addresses the need for improved understanding between ICT security and power experts. It explains important terms used differently in the two disciplines, identifies main impacts on power systems that may result from ICT incidents, and proposes a set of indicators that can be used as a basis for selecting measures.