Energy security is one of the main targets of energy policy. However, the term has not been clearly defined, which makes it hard to measure and difficult to balance against other policy objectives. We review the multitude of definitions of energy security. They can be characterized according to the sources of risk, the scope of the impacts, and the severity filters in the form of the speed, size, sustention, spread, singularity and sureness of impacts. Using a stylized case study for three European countries, we illustrate how the selection of conceptual boundaries along these dimensions determines the outcome. This can be avoided by more clearly separating between security of supply and other policy objectives. This leads us to the definition of energy security as the continuity of energy supplies relative to demand. If security is defined from the perspective of private utilities, end consumers or public servants, the concept could further be reduced to the continuity of specific commodity or service supplies, or the impact of supply discontinuities on the continuity of the economy.