The vast majority of current scientific research devoted to the phenomenon of solitude is performed within the framework of psychology, sociology, and literary criticism. However, these inquiries deal with specific manifestations of the multidimensional phenomenon of solitude, eliminating its nature and fundamental philosophical and anthropological essence, which is hidden behind secondary layers of empirics. The task of this investigation is to consider the phenomenon of solitude among representatives of the world’s philosophical thought of modern times, in whose teachings it has received the fullest expression, in particular among representatives of existentialism, philosophy of life, phenomenology, and psychodynamic theory, including K. Jaspers, N. Berdyaev, M. Buber, M. Heidegger, E. Levinas. Despite the noticeable outburst of engagement with the phenomenon of solitude in the philosophy in the mid 19th – mid 20th centuries, interest in it did not disappear, as evidenced by current works designed to bridge the gap identified by predecessors. Despite the differences in views on the nature of solitude and its impact on the personality, as well as on the moment when a person encounters their solitude, these thinkers have something in common – they all necessarily consider solitude through the lens of the Other (be it a specific individual or a whole society) and recognize it a necessary condition for both true knowledge of things and self-knowledge. Regarding the discrepancies in views on solitude, they may be caused by the “terminological chaos” that still prevails when considering this issue. The terms “seclusion”, “alienation”, “isolation”, “loneliness”, and actually “solitude” are used to denote various manifestations of this phenomenon, often without distinguishing them; classifications of solitude are built through descriptive definitions. The lack of clarity should be overcome by distinguishing related but different concepts and developing a unified approach to the relationship between the essence of the concept and its various lexical expressions. Such a clarification is achieved in this article by identifying commonalities and indicating differences in the works of modern philosophers, who were most interested in the phenomenon of solitude, assigning it an important place in their own philosophical explorations.