I remember the Real World Question debate (and we still have problems with it). The basic issue was this: do we want to become the dumping ground for every question every other stack is unwilling to answer?
While we're willing to embrace a great deal, a fair number of us clearly stated, "no!"
The Internet already has forums for anything-goes Q&A (Reddit, Quora, etc...). We want a Stack with standards. And while we're still striving for perfection, we've done a pretty good job creating reasonable limits.
As you read through this list your knee-jerk reaction may be, "that's not a list defining 'worldbuilding,' it's just a list of our Stack's expectations and rules." You're absolutely correct! And you're absolutely wrong. What we consider "worldbuilding" is very much limited by the rules.
- Is it your world?
We already have policies in place about whether or not a querent has the right to ask questions about a world that isn't their own. If you're asking about somebody else's world, or even the Real World, it's not your world and therefore inappropriate.
This should be distinguished between a real-world question asked in an effort to help build a fictional world of your own creation vs. a question about the real world for the purpose of better understanding the real world. Yup, it's a royal pain in the patootie to clearly discern the difference—but it does and should exist.
If you're asking a question about a world that is not yours, it's not worldbuilding on this Stack, unless a specific exception has been declared in Meta.
- Is the question specific?
We live with the reality that we have both our set of rules and Stack Exchange's set of rules. Stack Exchange is not a discussion forum. Ideally, we're helping people solve specific problems in their worldbuilding efforts. Practically, we entertain questions of a worldbuilding nature even if no world is presently being built. But in either case, we don't allow brainstorming or infinite list of things questions.
If you're asking for help overcoming a specific problem, or overcoming a specific lack of knowledge, that is inhibiting your ability to build a world, that's worldbuilding.
- Are you asking about your world... or your story?
We draw the line at story-based items vs. world rules that are universally applicable to any story. As an example, "Given the following list of gases, particles, and stellar conditions that make up and affect my atmosphere, what color will my atmosphere be?" is on-topic. "My community uses rocks for money and they paint them, what color paint should they use?" is off-topic.
Asking about your story is never worldbuilding. We're here to help you build a world, not tell a story.
- Finally, we're supposed to be more than physics-lite.
We certainly do get a lot of real-world questions! But that's had a consequence—too many people want to restrict answers to real-world physics. What's funny about this is that our understanding of physics constantly changes! And yet people want to believe (almost with a religious fervor) that today's understanding of physics or science is all the understanding there is or will ever be and therefore all answers to any question must meet that limitation.
Worldbuilding is so much more than can be justified by the limitations of science. But the opposite is true, a great many people want to embrace the fictional, the silly, the outrageous! But for some reason, they want that whimsy to be realistic. Too many people don't understand the difference between realistic (which can almost never be answered, "yes!") and capable of suspension of disbelief (which can almost always be answered, "yes!").
Worldbuilding embraces imagination, creativity, and a certain feel that can only be described by the word "maverick." Worldbuilding is an act of imposing structure and order where before there was only chaos. It embraces the whimsical equally with the punctilious. Sometimes the goal is to force that whimsy into the limits of the Real World. But at all times our goal is an answer that allows others to suspend their disbelief.
So, what is worldbuilding?
On this site, worldbuilding is any creative act that later permits a story to be told in any form (your alternate-universe simulator is just another way to tell a story). But we do limit that act. For the sake of standards and to appease our Stack Exchange overlords, we have rules. And our definition of worldbuilding necessarily sits within those rules.