Irreligion is the neglect or active rejection of religion and, depending on the definition, a simple lack of religion.

Irreligion takes many forms, ranging from the casual and unaware to full-fledged philosophies such as atheism and agnosticism, secular humanism and antitheism. Social scientists tend to define irreligion as a purely naturalist worldview that excludes a belief in anything supernatural. The broadest and loosest definition, serving as an upper limit, is the lack of religious identification, though many non-identifiers express metaphysical and even religious beliefs. The narrowest and strictest is subscribing to positive atheism.

According to the Pew Research Center's 2012 global study of 230 countries and territories, 16% of the world's population does not identify with any religion. The population of the religiously unaffiliated, sometimes referred to as "nones", has grown significantly in recent years. Measurement of irreligiosity requires great cultural sensitivity, especially outside the West, where the concepts of "religion" or "the secular" are not always rooted in local culture.