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From the Latin root word ligio, which in English became 'ligament', the term means literally 're-binding'. As such, religion is something practised by people who believe that they are separate from ultimate reality, and wish to do something about it. Religion is generally something people partake in so that they can feel important.
Religion usually catches the blame for many of recorded history's most gruesome wars and acts of gratuitous cruelty. There is some validity to this, but it is perhaps worth noting that most religions actually discourage this sort of thing. Religion and politics tend to provide a readily available justification for such ventures.
Most religions encourage people to follow the 'golden rule', that is, to do unto others as you would have others do unto you. That sounds great, but many people would be rather perturbed to encounter a masochist who followed that principle closely.