The basic question to be answered in this thesis is: Is there a way to reach a consensus about a generally accepted definition of terrorism by using the perspectives of history and legitimacy in the World Community? The solution to the problem about the definition of terrorism is an important question because of the sensitivities displayed by the nations of different regions and their different perceptions of terror. The questions about the nature of the terrorists, their motivation, their aims and the methods they use to achieve these aims are still being debated world-wide. On the other hand, terrorists also have the ability and possibility to reach the same sources regarding history and legitimacy; therefore it may be thought that, they have a variety of options about learning how to become more deadly without taking all the population of the target community to the opposite side. To fight terrorism effectively, what is needed first is to know who the enemy is. In this thesis, the answer to this question is carefully analyzed and a solution is offered. Brief information about the background of terror events in history and jurisprudence concerning national and international community is added to help understand the subject. Instead of putting up a certain and complete definition that may lead to many misunderstandings and a danger of constriction of the concept, the preconditions to make an efficient definition of terror is explained. This is important because there are certain difficulties in discriminating terrorists from other types of criminals, especially guerilla fighters and organized criminals. In this thesis, the purpose of the guerilla activities and their methods, also the differences and common points of these two are analyzed and explained. As an accessory, the statistics of terror events between 1973- 2003 are included in the thesis.