Digest of terms
This is an omnibus term with a myriad of alternative definitions. Classically, it is used to refer to identifiable traditions - Buddhist, Christian, Confucian, Hindu, Jewish, Muslim, Shinto, Sikh, Taoist, Zoroastrian and many others. Each of these may have its own self-definition - and probably several. Social scientists warn against the dangers of 'reification' such that the experience which has actually to do with quite dynamic personal feelings, convictions and activities is endowed with a concrete form which is actually a distortion because artificially 'thingified'. Some religious respondents say 'ok there are enormous variations, but there are also recognisable family resemblances' which are the basis of being able to claim a specific religious identity.
How such family resemblances are characterised across religions is contentious. Belief in God (theism) would be seen by some as a fundamental feature, but this would be challenged in the Confucian tradition and by Theravadan Buddhists, and though apparently a contradiction in terms there are well documented examples of Catholic and Jewish atheists. Possession of a single book of revealed scripture would be expected by some, but disputed by others - eg African Traditional Religionists and Hindus. And similarly, prescriptive rituals may or may not be fundamental.
Claims that there is a common core in religions are often advanced in such comments as 'they all share a commitment to the Golden rule of loving your neighbour' or 'deep down there's a sense of spiritual oneness'. But closer scrutiny is cautionary of claiming too much.
Abandoning a search for 'the essence of religion', some scholars have instead preferred to map component features common to religious experience. 'Behaviour, belief and belonging' is one such characterisation. 'Creed, code, cult' another. A map especially influential in the UK is a dimensional analysis, comprising doctrinal and philosophical, mythic and narratival, ethical and legal, ritual and practice, experiential and emotional, social and institutional, material and symbolic. These are common dimensions though their content is hugely diverse and their relative importance variable from one religion to another.
Another mapping overview identifies differences of emphasis in some traditions as compared with others. Typically, it is observed that in some religions an emphasis on inward mystical experience predominates (so Buddhism and Hinduism), in others a prophetic justice (eg Islam and Judaism) and in yet others wisdom (Confucian and Taoist). However, that does not preclude the existence also of the other elements, albeit less centrally within the same religion.
As a counter to the view that religions are optional extras layered on top of secular human experience, some theologians (eg Tillich) suggest that by definition they have to do with the most ultimate and far reaching, the deepest and most intimately held of all concerns. As such they are universal for all humans, even if they do not fit easily into the category of a specific religious tradition. The educationalist Phenix systematises this to suggest that religion coincides with the final and most complete sense of human meaning. As such yet others prefer the alternative term of 'world view' as less 'provocative'.
An advantage of inclusive definitions is that comparative light is shone on the structure and passions of other human collective activities, such as football following, or the Long March of Mao. By contrast a disadvantage arises from feelings of resentment at 'being converted by definition'. And as to the relation between religion and ethics, so much evidently does depend on definition.