Religion is a term that has been given to a wide variety of social formations throughout the world. It is a concept that scholars have discussed at great length, producing a staggering array of definitions. Some of the most important approaches have been monothetic, arguing that there is an essential property that all instances of religion must share; others have been functional, arguing that certain types of activities can only be described as religious. More recently, there has been the emergence of polythetic definitions that are based on prototype theory.

Substantive definitions of religion tend to define it as people’s relation to that which they consider to be holy, sacred, or ultimate; spiritual, divine, or worthy of especial reverence. This may involve the belief in a particular god or spirit, the worship of deities and saints, the veneration of texts or objects or natural forces, rituals and sermons, feasts, initiations, matrimonial and funerary services, trances, and various other practices.

One of the problems with substantive definitions is that they tend to impose their own assumptions about what is real or important about a phenomenon in order to determine whether it is a religion. For example, the beliefs of some people may include a belief in disembodied spirits or cosmological orders that are not accepted by most others. This can lead to a view that these are not religions because they do not have an afterlife or explicit metaphysics.

In contrast, some functional definitions of religion are more flexible and broader in their scope. For example, Emile Durkheim suggested that religion could be defined as whatever system of practices unites a number of people into a single moral community (irrespective of their belief in unusual realities). Such a definition can accommodate a wide range of religious beliefs and behaviors, a fact that makes it particularly useful for comparative study of forms of life across cultures.

Other definitions of religion are based on analogical or family-resemblance criteria. These rely on the idea that there are a number of things that can be called religions, but that these have no common essence; they differ only in their degrees of analogical similarity. This approach allows for considerable diversity among the different religions and also accommodates new forms of religion that develop as people discover new ways to organize their lives and express their beliefs.

Other critics of the notion of religion have gone so far as to argue that there is no such thing as a religion at all. This is a form of anti-reductionist critique, arguing that it is wrong to reduce all of the diverse phenomena in human society to any single concept such as religion. It is important to note, however, that the emergence of concepts for social kinds does not necessarily wait for the development of language; a social kind may be labeled by a term even if it has never been communicated in written terms. In this way, the word religion has been used for over two thousand years to describe a social phenomenon.