For questions that require plausible (better than suspension-of-disbelief) answers based on Real World science that are not necessarily constrained to the known limits of Real World science. Contrast with the hard-science, science-fiction and internal-consistency tags. This tag may not be used alone. This tag may not be used with the science-fiction, hard-science, or internal-consistency tags.
Similar to questions using the science-fiction or hard-science tags, questions tagged science-based expect answers related to science as understood by humanity at a specified time (and, if possible, in a specified place) in human history. However, unlike the science-fiction and hard-science tags, the science-based tag expects answers to be plausible (something more than just suspension-of-disbelief) or realistic.
The reality-check tag is now a synonym of the science-based tag, because questions asking for a "reality check" are fundamentally asking if the proposed idea reflects the plausible limits of known science. AKA, "is my idea science-based?"
Questions with this tag should be answered, as far as possible, using known scientific facts or reasonable extrapolations from known scientific facts. The answers are not required to provide scientific citations or be held strictly to rigid scientific interpretation. For example, we do not currently have the material science required to build a space elevator, but we can discuss the potential risks, consequences and benefits of building one based on known scientific facts.
When discussing future technology it is harder to know where to draw the line, but answers should still be rooted in what we know of the universe and be compatible with the current state or a reasonably extrapolated state of scientific knowledge. Questions using the far-future tag should consider using the science-fiction tag. The more we must extrapolate the potential limits of science, the more we're embracing fanciful, creative, and imaginative uses of science.
Most answers on Worldbuilding are expected to be based on logic and, to some degree, science by default, so even questions without this tag may receive scientific answers. However, the use of this tag indicates that the user specifically expects science-based answers; answers based on magic or pseudo-science are not valid and may be deleted.
This tag frames the answer, not the question. As such, it cannot be the only tag attached to the question. One or more subject-specific tags mus be included or the question will be closed as needing more details.
If you want a fanciful or imaginative answer based on Real World science (vs. magic or the pure invention of world rules, aka, a "plausible" or suspension-of-disbelief answer), use the science-fiction tag.
If you want only scientifically supported answers, including citations, meaning the answers must be factual Real World solutions to the problem that definitively exceed suspension-of-disbelief, use the hard-science tag.
If you want to test an idea, assertion, condition, situation, circumstance, or application of rules against the rules of your fictional or imaginary world, use the internal-consistency tag.
For details concerning the assignment of the reality-check tag as a synonym of the science-based tag or the conversion of the original reality-check tag to its new name, the internal-consistency tag, see the following meta post: If we can't burn the "reality check" tag, can we rename it?