1. the belief in and worship of a superhuman controlling power, especially a personal God or gods.
    “ideas about the relationship between science and religion”
    • a particular system of faith and worship.
      plural noun: religions
      “the world’s great religions”
    • a pursuit or interest followed with great devotion.
      “consumerism is the new religion”

At the first glance, it looks obvious, but if you give it a second thought, it may be a controversy worthy of entire courses, graduate theses, etc. So let’s try to define what makes a religion, religion. 

In my opinion, there are three major characteristics of religion:

  1. Religious practices (may include rituals, sermons, commemoration or veneration (of a deity, gods, or goddesses), sacrifices, festivals, feasts, trances, initiations, funerary services, matrimonial services, meditation, prayer, music, art, dance, public service, or other aspects of human culture)
  2. Worldviews and beliefs (faith, in addition to reason, has been considered a source of religious beliefs)
  3. Social organisation