During the bipolar period, insurgencies were most often considered as a way of fighting for liberation from colonization, repression and other forms of political violence. However, its features as a foreign policy tool were also noticed, so at the unipolar geopolitical moment they gained greater importance. Due to such a variety of manifestations, many definitions of this complex form of political violence have emerged. The problems that researchers encounter in searching for the essential features of the concept of insurgency are semantic and substantive because it is difficult to define what insurgency is, in relation to what it is not. However, the definition of this (and every other) social phenomenon is necessary because it defines its main and constant features, structure, as well as the differences from other similar phenomena. In this way, preconditions for a common approach and joint action in solving social problems are provided. In our search for a valid definition of insurgency, the characteristic features in dictionaries, encyclopedias and lexicons have been critically compared. Also, legal (in national and foreign statutes/laws and in international humanitarian law), military (doctrinal) and research definitions have been analyzed, in order to consider the problem of defining this concept and submitting a proposal for an objective and explicit definition. After analyzing the concept of "insurgency" it can be concluded that insurgency in our lexical discourse is "an armed way of expressing dissatisfaction or disagreement with the policy pursued by the government". Besides motive, doctrinal documents emphasize other important characteristics of this phenomenon, primarily violence, political basis, illegality and its (at least initially) illegitimacy. The genus proximum, or the first term of a higher order in relation to insurgency is political violence, and insurgency differs from other forms of political violence in terms of complexity, illegality, mass and other characteristics. Recognizing nonnegativity, objectivity, content, essential precision, complexity and accuracy as features of a good definition, researchers have differently defined this social phenomenon, but it can be noticed that available definitions only partially meet the mentioned criteria. Some of the definitions emphasize goal or motive (taking over political power, overthrowing the legal order, mastering resources, implementing party policy, changing the constitutional order, etc.). Others emphasize activities or method (subversion, military pressure, coup, etc.). The third group of definitions emphasizes the features of this phenomenon (unconstitutional, violent, ideologically motivated, etc.). Based on the abovementioned, it can be concluded that insurgency, as a complex form of political violence, is difficult to define precisely, so that its definition achieves a broad consensus. Definitions evolve depending on the context in which insurgency took place or is taking place, as well as the political goals and method chosen by insurgent movement or its sponsor. Based on the analysis of available definitions of insurgency and due to the constant change of categorical concepts, different approaches in defining and researching insurgency, an operational explicit definition can be formulated: insurgency is illegal intrastate complex form of political violence used by organized armed groups in order to achieve political goals. We expose this definition to professional and scientific criticism, stating that the problem of defining the concept of "insurgency" will continue to change due to development of this form of political violence because goals, activities, means and methods of insurgent organizations and contexts in which insurgency happens will also permanently change in accordance with current and future socio-political conditions, as well as motives and interests of key actors in this complex form of political violence.
