This week in New York, another 20th anniversary commemoration related to the Sept. 11, 2001, terrorist attacks in the United States will take place -- the U.N. Security Council's adoption of Resolution 1373 on Sept. 28, 2001. With far-reaching impact on the past two decades of counterterrorism practice, it imposed a set of legal obligations on all countries to take action against terrorism and established a Security Council body - the Counter-Terrorism Committee (CTC) - to monitor implementation of the resolution around the globe. In addition to catalyzing a wide range of national counterterrorism activities, it served as the foundation of what has become a sprawling international counterterrorism architecture. Yet, it did all of this without ever defining who is a terrorist.

Speaking in the weeks following the 9/11 attacks, the CTCs first chair, U.K. Ambassador Jeremy Greenstock proclaimed: "Increasingly, questions are being raised about the problem of the definition of a terrorist. Let us be wise and focused about this: terrorism is terrorism ... What looks, smells and kills like terrorism is terrorism."