Here is Merriam-Webster Dictionary Definition:
Main Entry: re·li·gion
Pronunciation: \ri-ˈli-jən\
Function: noun
Etymology: Middle English religioun, from Anglo-French religiun, Latin religion-, religio supernatural constraint, sanction, religious practice, perhaps from religare to restrain, tie back — more at rely
Date: 13th century
1 a : the state of a religious <a nun in her 20th year of religion> b (1) : the service and worship of God or the supernatural (2) : commitment or devotion to religious faith or observance
2 : a personal set or institutionalized system of religious attitudes, beliefs, and practices
3 archaic : scrupulous conformity : conscientiousness
4 : a cause, principle, or system of beliefs held to with ardor and faith
— re·li·gion·less adjective
http://www.merriam-webster.com/dictionary/RELIGION