Religion has many different meanings depending on what culture one comes from.
The anthropology of religion perhaps began with Al Biruni (973 1048), an Islamic scholar from Persia, who wrote detailed scientific studies of religions and cultures across the Middle East, Mediterranean and the Indian sub Continent. He is accorded as the founder of anthropology.
Anthropologists have looked at religion down the ages and examined the symbols that have been worshiped, the deities being glorified, the temples, shrines, churches and even the nature of the gods being promoted. There has been a thousand year history in this study. Anthropologists have further looked at the moral codes, the ethical codes, the organizational hierarchies of different religions to find a universal definition. While there are folk religions, shamanistic religions, major Indian religions, Abramatic religions, Taoic religions and many others, for some reason, there is not a single explanation that truly defines what religion is. The dictionaries have not any good definitions.
So, let us take one step back again. Who is it that is seeking these religions. We are. Then what are we? With that question answered then perhaps we can see what man is attempting to do with all these religions. And as the number of religions likely range into thousands and maybe into the millions, it is unlikely and irrational that only one religion is right, and the others are simply wrong. So a definition would have to be all covering.
As said, dictionaries are not that useful here. Even older dictionaries. They tend to concentrate on deities, systems of worship and so on. But the derivation of the word religion is a good clue. It is Latin, and re meaning back, and ligare, meaning to bind. It implies that religion is a phenomena to bind us back to something.
So what is life? Life is that entity-force which we answer to as I. It has properties and follows certain laws that govern its existence and operation in the physical universe.
Religion could then be described as a philosophy of knowledge on how the life force, which we claim as I and me, exists, came to be in, operates or inter-relates within, the physical universe and how each life-force relates to each other, and the definition further would include how that life-force relates to not only the physical universe but to the allness or all, the absolute everything of everything including the full expanse of this and any other universe, and the universal creator of creations.