The concept of social solidarity is one of the fundamental notions in classical social theory. The paper presents a study of this concept development in famous sociologists' works (O.Compte, H.Spenser, F.Tönnies, E.Durkheim, and K.Marx) as well as in modern Ukrainian, Russian, European and American social scientists' researches. The authoress develops a theoretical definition of the concept 'social solidarity' by drawing on three approaches to the notion definition. These are (1) attributes distinction, (2) explanation of the functions and (3) description of relationships between this concept and other concepts of social theory. As a conclusion, a general definition of 'social solidarity' is presented as an important conceptual precondition for further development and relevant usage of the concept in empirical studies.