The Interlocutory Decision on the applicable law handed down on 16 February 2011 by the Appeals Chamber of the Special Tribunal for Lebanon revived the debate about the still controversial question of whether a generally accepted definition of terrorism as an international crime exists. This paper, after having considered the most debated points of the terrorism definition, provides a commentary on two specific terrorism-related aspects of the decision: (i) the court’s finding that terrorism has crystallized to form a distinct international crime under customary international law and (ii) the consistent expansive approach to the interpretation of Lebanese law which provides the basis for a nullum crimen sine lege challenge, given the clear legislative intent to apply the more restrictive Lebanese form of the crime of terrorism. This contribution is intended to answer questions such as: (i) what kind of contribution (if any) has the STL made to the progressive development of a definition of terrorism as an international crime?; (ii) is there room for doubt about its compliance with the legality principle, and more generally (iii) does respect for this principle stand in the way of the development of International Criminal Law?