Secularism is the principle of seeking to conduct human affairs based on naturalistic considerations, uninvolved with religion. It is most commonly thought of as the separation of religion from civil affairs and the state and may be broadened to a similar position seeking to remove or to minimize the role of religion in any public sphere. Secularism may encapsulate anti-clericalism, atheism, naturalism, non-sectarianism, neutrality on topics of religion, or antireligion. Secularism is not necessarily antithetical to religion, but may be compatible with it. As a philosophy, secularism seeks to interpret life based on principles derived solely from the material world, without recourse to religion. It shifts the focus from religion towards "temporal" and material concerns.

There are distinct traditions of secularism like the French, Turkish, American and Indian models. These differ greatly, from the American emphasis on avoiding an established religion and freedom of belief, to the French interventionist model, and more. The purposes and arguments in support of secularism vary widely, ranging from assertions that it is a crucial element of modernization, or that religion and traditional values are backward and divisive, to the claim that it is the only guarantor of free religious exercise.

Both "religion" and "secular" are Western concepts that are not universal across cultures, languages, or time; with experiences of secularism varying significantly. Secularism has origins going back to the ancient world into religious texts such as the Bible, being refined through history by religious thinkers. Secular individuals hold complex relations to religion.

Secularism takes different forms with varying stances on where and how religion should be separate from other aspects of society. People of any religious denomination can support a secular society, or adopt the principles of secularism, although secularist identity is often associated with non-religious individuals such as atheists. Political secularism encompasses the schools of thought in secularism that consider the regulation of religion by a secular state. Religious minorities and non-religious citizens tend to support political secularism while members of the majority religion tend to oppose it. Secular nationalists are people that support political secularism within their own state.

Scholars identify several variations of political secularism in society. The strictest form, associated with the French laique model, advocates a state that is both firmly and officially distanced from all religions and non-religious philosophical convictions in all of its manifestations and official dealings, without exception. A more "humanistic" form is indifferent towards religions per se but also advocates for the states to operate on purely a rational basis of evidence-based policy and a focus on human needs and welfare, entailing non-discrimination between peoples of differing religions and non-religious philosophical convictions throughout society. A third "liberal" or "pillarized" form of secularism holds that governments may in some instances express sympathy to, provide funding to, license state services to, or otherwise allow unique special treatment of religions (common in German-speaking and Benelux secular states), so long as states nevertheless treat these convictions equally, and are neither hostile nor preferential towards any particular set of religious or non-religious philosophical convictions such as humanists. In these countries, secular humanist organizations typically receive state funding according to the same funding formulas used to provide state funding to religious groups. In Indian political discourse, the pejorative term pseudo-secularism is also used to highlight instances where it is believed that while the state purports to be secular, indifferent, or impartial towards religions, its policies in reality favour a particular religion over others.

There are many principles that are associated with all forms of political secularism. It typically promotes legal equality between people of different religions, opposing a legal hierarchy on the basis of religious belief or lack of religious belief. It is also associated with a separation of church and state, considering these to be two distinct entities that should be treated separately. State supremacy is a secular principle that supports obedience to the rule of law over religious diktat or canon law, while internal constraint is a secular principle that opposes governmental control over one's personal life. Under political secularism, the government can enforce how people act but not what they believe. Similarly, freedom of thought is supported by secularism. Order is supported by secularists, specifically in that one's beliefs should not be permitted to disturb the civil peace. Religious tolerance is supported both for people of other religions and for a lack of piety demonstrated by members of one's own religion. Political secularism also supports reason as a virtue. Secularists also support freedom from religion as an extension to freedom of religion.

Secularism and "secular" is a Western concept that is not universal across cultures, languages, or history. The term "secularism" was coined in 1851 in Britain. In societies such as Ancient Greece, a limited secularism was practised in which religion was not involved in governance, though it was still prevalent in public life.

Secularism's origins can be traced to the Bible itself and fleshed out throughout Christian history into the modern era. The "Secular" is a part of the Christian church's history, which even has secular clergy, since the medieval period. The distinction between secular and religious law was emphasized in the Late Antique and early medieval West. Secular and religious entities could be distinguished in the medieval period, but coexisted and interacted naturally. Significant contributions to principles used in modern secularism came from prominent theologians and Christian writers such as St. Augustine, William of Ockham, Marsilius of Padua, Martin Luther, Roger Williams, John Locke and Talleyrand.