For questions about "hard" science fiction, use on [story-identification] questions when the work is hard sci-fi and for questions about hard sci-fi in general.

Science-fiction is often separated into "hard" and "soft", where "hard" sci-fi is science-fiction that follows the scientific models of the universe (e.g. physics) that are understood at the time of original publication.

Examples of when this tag is appropriate:

Questions that ask whether a particular work is considered "hard" are not currently considered on-topic for this site.

There is a more detailed explanation about how to distinguish between "hard" sci-fi and other sci-fi in a question on this site.