The category of propaganda is considered in the paper as a linguistically definable notion. It is demonstrated that being a legal term, this category is significantly influenced by the semantics of the word propaganda as a unit of the Russian literary language. Presented is an analysis of various definitions of propaganda in their historical evolution, from the earliest record of the word in Russian dictionaries to modern lexicographic definitions. Polysemy of the lexical unit “propaganda” is analyzed. Special focus is placed on differentiation of propaganda from other forms of persuasive communication, particularly from advertising, including its political variety. Analysis of the meaning and use of the word propaganda is proposed as a way to objectivize the intuitive labeling of various statements as propagandistic. Our analysis resulted in a list of key properties of propagandistic discourse to be taken into account in the investigation of suspected propaganda of extremism and terrorism, namely, the existence of: (1) a subject of propaganda; (2) an addressee of propaganda, (3) a set of relatively simple and consistent points (describing the desired state of affairs) and supporting arguments; (4) opposing evaluations of the desired situation and its negative alternatives; (5) recurrent and persistent realization of the aforementioned set of points or its fragments in the discourse, with identical of slightly modified argumentation; (6) multiple texts produced within a certain period of time (i.e. the period of a propagandistic campaign); and (7) a practical possibility for disseminating information among multiple people (addressees).