Dictionaries are common use. Which is fine for everyday use, lay use and so on. But it falls short of how terms are used in philosophy and theology, as well as it's etymological origin. (For example, atheism used to not mean without belief in gods because that's not what theism was. Theism was specifically the belief in a god who was governing the universe, which would have made deism atheistic.)
It also leaves out other terms not part of the common vernacular, like ignostic.
Nor does the dictionary do a very good job (as far as philosophy is concerned) with differentiating ideas under the atheistic umbrella, such as strong atheism vs soft atheism, implicit vs explicit atheism, Gnostic vs agnostic atheism.
But yeah, it will give you the ball park common use definition.