"A set of beliefs and practices that are subscribed to by one or more people that involves the worship of an entity (or entities)."
Dictionary.com
Wiktionary:1.
a. Belief in and reverence for a supernatural power or powers regarded as creator and governor of the universe.
b. A personal or institutionalized system grounded in such belief and worship.
2. The life or condition of a person in a religious order.
3. A set of beliefs, values, and practices based on the teachings of a spiritual leader.
4. A cause, principle, or activity pursued with zeal or conscientious devotion.
M-W:1. A system of beliefs that involves the existence or nonexistence of at least one of: a human soul or spirit, a diety or higher being, or self after the death of one's body.
* He couldn't abide by any religion that didn't allow for wrongdoers to be punished after death.
2. Any system of beliefs.
Note: this usage is uncommon, see Usage note, below.
* Science is a religion, because it relies on certain fundamentally unprovable ideas, such as the existence of logic.
3. A system of belief predicated on the existence of one or more dieties.
Note: this usage is uncommon, see Usage note, below.
* It's not really a religion if there's no god to pray to.
4. A way of living that corresponds to such beliefs.
* You can practice any religion you like, as long as it doesn't require you to violate our laws.
5. A number of customs and rituals associated with such beliefs.
* When it comes to religion, she doesn't believe, but she loves to attend the ceremonies.
6. Anything that involves the association of people in a manner resembling a cult.
* At this point, Star Trek has really become a religion.
7. Any system or institution which one engages with in order to foster a sense of meaning or relevence in relation to something greater than oneself.
New Advent: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
CARMReligion, broadly speaking, means the voluntary subjection of oneself to God.
Generally a belief in a deity and practice of worship, action, and/or thought related to that deity. Loosely, any specific system of code of ethics, values, and belief.