Solidarity or solidarism is an awareness of shared interests, objectives, standards, and sympathies creating a psychological sense of unity of groups or classes. True solidarity means moving beyond individual identities and single issue politics. Still, solidarity does not reject individuals and sees individuals as the basis of society. It refers to the ties in a society that bind people together as one. The term is generally employed in sociology and the other social sciences, as well as in philosophy and bioethics. It is a significant concept in Catholic social teaching and in Christian democratic political ideology. Although closely related to the concept of charity, solidarity aspires to change whole systems, not merely to help individuals.

Solidarity is also one of six principles of the Charter of Fundamental Rights of the European Union, and International Human Solidarity Day is recognized each year on December 20 as an international observance. Solidarity is not mentioned in the European Convention on Human Rights nor in the United Nations' Universal Declaration of Human Rights and therefore has lesser legal meaning when compared to basic rights.

Concepts of solidarity are mentioned in the Universal Declaration on Bioethics and Human Rights, but not defined clearly.

The terms solidaire and solidairement had already appeared in French legal language in the 16th century. They are related to the Roman legal concept in solidum, which was derived from the Latin word solidus, meaning "on behalf of the whole". In the Napoleonic code, solidarity meant the joint liability of debtors towards a common creditor and was not a primary legal principle.

Conservatism, following the French Revolution, introduced the concept of "solidarity", which was detached from the legal system, as a reaction against rapid social change and as a longing for a stable society. During the July Monarchy, Pierre Leroux, a utopian socialist who is also said to have coined the term socialism, also introduced the concept of non-legal solidarity. Auguste Comte, the so-called founder of sociology, adopted the concept in the sense of social interdependence between people. Comte linked solidarity to the concept of altruism as the opposite of egoism. Instead of emphasising the individual, altruism emphasises common responsibility and solidarity. The interpretations of Pierre Leroux and Auguste Comte gave rise to the idea of a specific social solidarity as the basis of the social order.

After the French Revolution, new scientific and ideological interpretations of solidarity emerged in France. The concept took on sociological, economic, legal, and political variants. Thinkers with different emphases shaped the meaning of the concept of solidarity to suit their own purposes.

The Paris Communards, for example, exchanged the revolutionary slogan of "fraternity" for "solidarity". Some French liberal economists also began to use the term "solidarity", but they changed its meaning in an individualistic direction. Liberalists argued that interdependence between people meant that people also had to take responsibility for their actions without the state intervening. Charles Gide, an economist who opposed liberalism, developed his own interpretation of the concept and even proposed solidarity as the name of a new school of economics.

Through these stages, by the turn of the 20th century, solidarity had become a generic term that could be associated with almost everything that was considered good and progressive. The Paris World Fair in 1900 was accompanied by a congress on "social education and the new solidarity". The Catholic Church also began to use the popular concept of solidarity. According to sociologist Steven Lukes, solidarity played a role in France at the time that was almost as strong and influential as individualism was in the United States at the same time.