The encyclical Sollicitudo rei socialis conceives the Catholic Social Teaching (CST) as an accurate formulation of a reflexion on the social problems, in the light of faith and the Church's tradition. This article explores the formulation of the principle of solidarity since it is hard to find a clear or complete definition of the principle itself in magisterial texts. The article harkens back to the complex origins of the principle of solidarity in CST and in the social sciences, and extracts the main contents pointed out by the CST. Next, the importance of counting on an accurate definition of the principle is explained and a definition is proposed in view of previous CST. Given the influence of the social sciences on the formulation of CST, the article finally highlights the convenience of purifying questionable elements when it comes to present and explain solidarity as a principle.