Before we can explore the ontology of security, we must establish a preferred meaning of the word and purge it, as far as possible, of ambiguity. This preliminary task is especially important when one’s subject is a term in common currency and deployed in a wide variety of contexts for a wide variety of purposes.1 The more familiar a word, the more likely we are to take for granted that our own particular understanding of it is widely shared and that its meaning goes without saying. This is as true of the word ‘security’ as it is for almost every other key concept in the study of world politics.