Secularism is generally the assertion that certain practices or institutions should exist separately from religion or religious belief. Alternatively, it is a principle of promoting secular ideas or values in either public or private settings. It may also be a synonym for "secularist movement". In the extreme, it is an ideology that holds that religion has no place in public life.

In one sense, secularism may assert the right to be free from religious rule and teachings, and freedom from the government imposition of religion upon the people, within a state that is neutral on matters of belief, and gives no state privileges or subsidies to religions. (See also Separation of church and state and Laïcité.) In another sense, it refers to a belief that human activities and decisions, especially political ones, should be based on evidence and fact rather than religious influence. (See also public reason.)

The purposes and arguments in support of secularism vary widely. In European laicism, it has been argued that secularism is a movement toward modernization, and away from traditional religious values. This type of secularism, on a social or philosophical level, has often occurred while maintaining an official state church or other state support of religion. In the United States, some argue that state secularism has served to a greater extent to protect religion from governmental interference, while secularism on a social level is less prevalent. Within countries as well, however, differing political movements support secularism for equally varying reasons.