In sociology, secularization (or secularisation) is a multilayered concept that generally denotes "a transition from a religious to a more worldly level." Most versions of secularization do not lead to atheism, irreligion, nor are they automatically anti-thetical to religion. Secularization has different connotations such as implying differentiation of secular from religious domains, the marginalization of religion in those domains, or it may also entail the transformation of religion as a result of its recharacterization (e.g. as a private concern, or as a non-political matter).

The term "secularization" is also used to mean the lifting of monastic restrictions from a member of the clergy, and to deconsecration, removing the consecration of a religious building so that it may be used for other purposes. Though the concept of secular comes from old Latin and was used up to the medieval period, the first use of the term as a change from religion to the mundane is from the 16th century that referred to transforming ecclesiastical possessions for civil purposes, such as monasteries to hospitals; and by the 19th century it gained traction as a political object of secularist movements. In the 20th century, "secularization" had diversified into various versions in light of the diversity of experiences from different cultures and institutions.

The secularization thesis expresses the idea that through the lens of the European enlightenment modernization, rationalization, combined with the ascent of science and technology, religious authority diminishes in all aspects of social life and governance. In recent years, the secularization thesis has been challenged due to some global studies indicating that the irreligious population of the world may be in decline as a percentage of the world population due to irreligious countries having subreplacement fertility rates and religious countries having higher birth rates in general. Christian sociologist Peter L. Berger coined the term desecularization to describe this phenomenon. In addition, secularization rates are stalling or reversing in some countries/regions such as the countries in the former Soviet Union or large cities in the Western world with significant amounts of religious immigrants. Even global studies show that many people who do not identify with a religion, still hold religious beliefs and participate in religious practices, thus complicating the situation.