Abstract: The subject of this research is the problem of understanding and interpretation of the meaning and designation of one of the key concept of modern legal lexicon – “legitimacy”. Legitimacy became an attribute of the current scientific paradigm of legal thinking, because broadening the area of application, it is used as a certain standard of highest legality, often perplexing comprehension of the problems of legal theory, as on etymological level its leans only on one of the Latin versions of the word “legal”. In the same platitude, legitimate legality and legal legitimacy are a common tautology, which in the context of theory of law and philosophy of law, insistently dictates the necessity to clarify this term and definition. Research methodology suggests the analysis of the problem of legality-legitimacy from the perspective of differentiating legal and other disciplinary approaches: political scientific, sociological, etc. The novelty of this study consists in the problematic-critical analysis of the concept of “legitimacy” on the context of theory of law and philosophy of law. The conducted research demonstrates that the problem of legitimacy represents is a terminological substitution within the framework of legal-positivistic doctrine for the so-called not “purely” legal aspects: sociological, psychological, political scientific, and other.