So, what's the difference? The difference is that Webster's current definition implies that a person actively chooses to believe something. Atheism is not a belief system. It is instead the lack of a belief system. It requires no action.
Anyone who does not believe in a deity is an atheist. This includes new born babies and people who have had no exposure the concept of deities. New born babies are atheists because they are not capable of understanding the notion of deities, let alone believe in any. It would not be accurate to say that they believe that deities do not exist. Yet, they are atheists.
Even the agnostic individual, who does not think it is possible to prove whether or not deities exist, would be an atheist. Why? Because that person cannot say "I believe that deities exist."
What about the tribes of people who exist in the most remote parts of the world who have never been exposed to the notion of deities? As with babies, you cannot say that they believe that deities do not exist. They have no concept of deities in the first place to believe they don't exist. They simply do not believe in the existence of deities.
To say that someone believes that no deities exist is to impose the act of rejection on the individual in question. Albeit, there are atheists who do actively reject the notion. However, that is not definitive of the word. That is more of a characteristic that certain atheists possess. But, not all have this characteristic.
Hopefully, we will see a change in the definition. I, for one, think it is important distinction in understanding the nature of religion and how people are affected by it. We shall see.
I will leave you with the response I received from Kory Stamper, Associate Editor at Merriam-Webster, Inc. Here it is:
Thank you for your message. A handful of other readers have also
written us about this, and we've been making careful note of all such comments
for review by our editors during the preparation of future editions.
Copyright 2008 Trina Hoaks