What exactly is atheism? Hilariously, atheists can't agree. When apologists address atheism as a whole, it's very common for atheists to claim that apologist doesn't understand atheism. Why? Because the definition the apologist uses disagrees with the definition they use.
There are two main problems with this. First, there are a great many dictionaries out there that include the word "atheism". These dictionaries typically apply a meaning somewhere along the lines of "the belief that God does not exist" (Cambridge Dictionary), "the doctrine or belief that there is no God", or "disbelief in the existence of a supreme being or beings." (both Dictionary.com). How do I generally define atheism on this ministry? Well, pretty much just that. As I like to say, if you have a problem with that, take it up with Cambridge.
Which leads us to the second problem: Some people genuinely would have quite the heated argument with Cambridge over the definition of atheism. Is atheism really the belief that God does not exist, or is it just, as some say, a "lack of belief in God"?
Let me ask an alternative question. Is bear a noun, or a verb? It doesn't take much thought to know that the answer is both. A bear is an animal, meaning bear is a noun. To bear means to support (e.g. bearing weight) or to carry/present (e.g. "the right to keep and bear arms").
So, first, we have the problem of dual definition. We can accept that atheism can mean the lack of belief in God without assuming anyone who uses the meaning "belief that God does not exist" is ignorant and doesn't understand atheism.
But second, atheists themselves will give conflicting definitions! See, the thing about atheism is it isn't a fixed religion. It doesn't have a holy book, it doesn't have traditional literature, the absolute best it has is a dictionary definition. Even that depends on which dictionary you happen to be using. But who can scold you for using the "wrong" dictionary? An arrogant swine, that's who! See, because atheism is not a religion, no two atheists have to agree on anything. The only thing a person needs to be an atheist is to not include a divine being in their worldview. An atheist can believe literally anything else. They can believe in ghosts, reincarnation, Evolution, Astrology, aliens, any number of weird and wonderful things, as long as they don't believe in God, they are an atheist. Atheists can even believe Jesus rose from the dead as long as they don't acknowledge Him as Lord, or credit God with His resurrection.
Therefore, the only possible way an atheist can say "you don't understand atheism because you used a dictionary definition instead of my personal definition" is if they also say "I am the ultimate atheist! Anyone who does not believe exactly what I believe is either not a real atheist, or is less of an atheist than I am!" You know, sort of like a religious person might do...