The purpose of the study is to identify fundamental elements in the structure of the definition of terrorism based on the application of the endogenous-exogenous approach, as well as determine their necessity and sufficiency when defining an action as a manifestation of terrorism. The research methodology is a retrospective analysis of the essence and content of the structure of the definition of terrorism. A holistic system-integrated approach makes it possible to identify and substantiate the key elements that are leading and led in the process of defining an action as a manifestation of terrorism. The application of the method of mathematical modeling makes it possible to build a mathematical model that describes the structure of the definition of terrorism. As the main results of the study necessary and sufficient elements of the definition of terrorism, their classification, which contributes to the appropriate definition of certain actions as a manifestation of terrorism, and the construction of a mathematical model that describes the structure of the definition of terrorism are identified. The results will provide significant assistance in organizing antiterrorist activities in the modern world and help build a system for countering terrorist threats of the 21st century. The use of the results of the study is also possible in the process of expert assessment of projects and existing regulatory legal acts in the field of antiterrorist activities. The novelty of the study and its results are determined by an original approach to identifying and classifying elements in the structure of the definition of terrorism characteristic of terrorism of the 21st century, dividing them into two groups: endogenous and exogenous, as well as a developed mathematical model describing the structure of the definition of terrorism, which makes it possible to establish the interdependence between an action and the attribution of this action to a manifestation of terrorism.