I'd like to stop this line of reasoning, it is not relevant to the topic.
An atheist in my proposal is defined by what they think not what someone does not.
An athiest here has deliberatley chosen to not believe in a god or gods, this implies absolutely nothing about those who have not considered the issues, and doesn't imply the label can be assigned to anyone else.
But it is relevant to the topic...
Otherwise how do you tie this to the penultimate line of your OP: "If you are not actively a Theist, you are passively an Atheist."?
Surely someone who has not thought about the issue is
not actively a theist, right? Therefore this penultimate sentence of yours implies that anyone not a theist, for any reason at all, falls under the label "atheist".
If you wish atheism only to be an active position (i.e. held only after due consideration) then I'm not sure the definition works, and re-reading the linked-web-sites "What is Atheism" - it mentions nothing about it needing to be an intellectual position.
"Lack of belief in gods" is as concise as you're going to get. And still people will argue about what it means to have a "lack of belief".
Requiring agnosticism to be the foundation of atheism is debatable (imho).
Requiring it to be a position only held once considered... certainly no-one could call themself an atheist without considering it (the way you can't not think about a pink elephant when reading the words). But surely you can refer to someone as atheist without that person needing to have considered it.
Maybe a definition such as
"A considered position of not holding the belief that god(s) exist, arrived at through (intellectual honesty /) personal lack of evidence."
This thus makes it a position one arrives at after consideration (and not available to those who have not considered it), and skirts the issue of whether one holds knowledge of the metaphysical issues to be fundamentally unknowable.
