Religions are central to many people’s lives. They shape beliefs, ethics and morality. They are important forces shaping society and politics, locally and globally. They give meaning to the world and are a source of strength in times of crisis. Religions answer big questions like why the world is as it is and what our place in it is.

The concept of religion has a long and complicated history. Once a term for scrupulous devotion, the concept shifted to something much more expansive. In its contemporary sense it is difficult to determine which types of activities or practices belong in the category, and which ones do not. This is partly because the definition of religion is constantly evolving. Several different definitions are used in the study of religion, each of which has its own particular strengths and weaknesses.

One approach to defining religion takes the classical view that a concept has a set of defining properties, which are shared by all the examples of that concept. These are called “monothetic” definitions. They are still widely used, but are increasingly criticized for failing to adequately capture the full scope of human experience.

A different approach to the problem of defining religion is to look at the functions that the concept can perform for an individual or group. Emile Durkheim, for example, argued that religious practice creates solidarity within a social group, even when it does not involve belief in unusual realities. Paul Tillich’s functional definition is another influential version of this approach.

Other definitions try to define religion in terms of the specific kinds of experiences that can be considered spiritual or supernatural. This is sometimes called a “substance” definition, and it is often used in conjunction with the functional definitions. However, some scholars argue that it is not possible to understand religion in terms of the experiences themselves without also considering the structures that produce them. This is the “structure/agency” debate, and it is one of the liveliest areas of the study of religion.

The problems posed by the search for adequate definitions of religion are complex and ongoing. Attempts to impose a single, univocal notion of the term run the risk of reducing all human experience to a lowest common denominator. At the same time, attempts to define the term in ways that allow for the study of very different phenomena can result in a definition that is so narrow that it becomes virtually useless as a tool for research. The challenge is to find a way of navigating these difficulties and finding a good definition that will serve as a useful basis for understanding human religiosity. This article will consider some of the major alternatives for a definition of religion and discuss how they compare with each other. It will then turn to an analysis of the impact that the various definitions have on the study of religion. It will conclude by arguing that it is better to work with a closed polythetic definition of the concept than with a monothetic one.