Hard science fiction is a term for works in the science fiction genre which apply a strict, rigourous, consistent scientific model for the universe. The term is formed by analogy to the distinction between the "hard" (natural) and "soft" (social) sciences - approximate ways of characterizing stories with a position on a scale from "softer" to "harder", not a binary classification. From http://en.wikipedia.org/wiki/Hard_scifi
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:
- Asking for story-identification of hard sci-fi.
- Asking questions about how particular hard sci-fi relates to currently understood physical laws.
- Asking questions about how the science of a story relates to current technology.
Questions that ask whether a particular work is considered "hard" are not currently considered on-topic for this site.