Social systems whose participants avow belief in a supernatural agent or agents whose approval is to be sought.
Other definitions of "religion" lose utility because they allow too many other human social constructs to be included. If looser definitions of religion are used, then new terms and labels will need to be developed to describe those religions that include superstitions of supernatural agents. Assuming those looser definitions, social constructs that most people wouldn't consider as "religions" now must be considered as such.
This might be useful in the grand scheme of examining human social systems and sociological behavior, but serves only to confuse those who aren't aware of the effort and want to delineate between their rituals at the First Baptist Church on Sunday and their rituals at the local horse track on Tuesdays.