Every dictionary has a definition of religion, but not every definition is equally good. Some are decent, but others are horrible. Sometimes older dictionaries have better definitions than newer ones, but more recent comprehensive dictionaries tend to have the better overall definitions of all. It's easier to understand what religion is if you understand the advantages and disadvantages of some of the more commonly cited definitions found in dictionaries.
1a. 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.
1a. the state of a religious b(1). the service and worship of God or the supernatural b(2). commitment or devotion to religious faith or observance
2a. personal set or institutionalized system of religious attitudes, beliefs, and practices
3. archaic: scrupulous conformity
4a. cause, principle, or system of beliefs held to with ardor and faith
Both of these online definitions are problematic because the present the central defining characteristic of religion as belief in God specifically or, in the more generous example from YourDictionary, whatever powers created the universe.
There is nothing substantive about the psychological or social aspects of religion, all of which play an important role in distinguishing religion from other belief system. There are references to practices, but they are presented in a circular fashion stating that religion is a set of religious practices really doesnt tell us anything.