Twenty-first century access to energy sources depends on a complex system of global markets, vast cross-border infrastructure networks, a small group of primary energy suppliers, and interdependencies with financial markets and technology. This is the context in which energy security has risen high on the policy agenda of governments around the world and the term 'energy security' has quietly slipped into the energy lexicon. The limited discourse about the nature of the term or its underlying assumptions has been totally eclipsed by an almost overwhelming focus on securing supplies of primary energy sources and geopolitics. An examination of explicit and inferred definitions finds that the concept of energy security is inherently slippery because it is polysemic in nature, capable of holding multiple dimensions and taking on different specificities depending on the country (or continent), timeframe or energy source to which it is applied. This 'slipperiness' poses analytical, prediction and policy difficulties but if explicitly recognised through definitional clarity, new levels of understanding will enrich the policy debate to deal with obstacles impacting on the constantly evolving nature of energy security. (C) 2009 Elsevier Ltd. All rights reserved.