The article dwells on the problem of the definition of terrorism. This term, though usually commonly understandable, is a legal term, the definition of which is not yet universally adopted. The threat of terrorism, taking upon different forms, is always relevant and the efforts to contain it continue. Thus the definition of the phenomenon and its elements is crucial in such efforts. The analysis is relevant as internationally this term is still not defined in one universal legally binding document and there are questions on its exact scope. The article explains two elements that are usually considered essential in defining terrorism. Furthermore, the article dwells on the EU efforts to contain the threat and in particular on its efforts to prevent terrorism – that is, actions taken to counter terrorist radicalisation.