Definition of religion:
"the belief in and worship of a superhuman controlling power, especially a personal God or gods"So, no atheism is not a religion, as it rejects belief in any sort of supernatural being or god.
That seems to be a common misconception among religious people.
Agreed.
Should our understanding of the word religion be complete, or incomplete? Naturally, the answer is complete. To have a complete understanding of a concept is to understand the varying connotation of particular terms.
Religion is defined the way you present it, but it is also defined as simply:
“A pursuit or interest followed with great devotion.”
This definition exactly describes atheism. Frankly, I would say my definition is possibly better: “a system of belief establishing worldview, and the behavior patterns derived from such belief.” Although I would insert something about devotion or strong belief to make it even better.