For questions that require answers based on hard science, not magic or pseudo-science, but do not require scientific citations. Contrast with the hard-science and reality-check tags. Do not use this tag as the only tag on a question because it describes how the question should be answered, not the topic.
Questions with this tag should be answered, as far as possible, based on known scientific facts or reasonable extrapolations from those, but answers are not required to provide scientific citations. 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 of scientific knowledge.
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 asker wants specifically science based answers; answers based on magic or pseudo-science on this question are not valid answers and may be deleted.
Do not use this tag as the only tag on a question. Instead, use it in combination with subject-specific tags.
If you merely want plausibility, go for the reality-check tag instead.
If you need scientific citations in answers, use the hard-science tag instead.