Atheism is typically defined as the disbelief in the existence of God.
True.
However, many (if not most) atheists are inclined to argue that "God" is a meaningless term.
Can you elaborate your understand of why they think that "God" is a meaningless term?
What aspect of "meaningless" do you mean?
Is that mean that the term "God" have no characteristics apply to it in that atheist's opinion therefor it's meaningless?
Or that atheist think that the term "God" is meaningless to him because he disbelieve in the existence of that "God", it's meaningless because the "God" can't control his life decisions and didn't give any meaning to his life?
If you are an atheist who has made such an argument, then I would argue that your "atheism" is meaningless because you apparently do not understand what exactly you are disbelieving.
A type of religion's believer:
My religion's God exist, you(non-believer/atheist) should follow my God's moral/law, here is my reason why i believe my God exist, (reason present and explain).
A specific type of atheist:
I don't believe in your or any other religion's God's existence because your or any other religion's reason/evidence is unconvincing to me.
The term "God" may consider to be a meaningless term for me as i don't believe in any God's existence, this any "God" plays no role and does not control my life decision and it give no meaning to my life therefor it's meaningless to me but i understand that it may be meaningful to some believer who believe in this any "God", i've no problem for their choice to follow their religion's God, but if when they imposes their religion's God's moral/law unto me, i reserved that i've my rights to disagree with them when and if i found their action's reason to be unreasonable to me.
My atheism is meaningless?
No, i simply lack the belief in any deities, there is nothing meaningless about my lack of belief in any deities.
I apparently do not understand what exacly i'm disbelieving?
No, i did understand what i'm disbelieving. What i'm disbelieving is the existence of many different "Gods" from many different religions. Those believer may have many characteristics to apply to their God's personality, i may disagree with their claims according to my reason if i find their reason to support their claims unconvincing to me.