Hard science fiction is a category of science fiction characterized by an emphasis on scientific accuracy or technical detail, or on both.
The heart of the "hard SF" designation is the relationship of the science content and attitude to the rest of the narrative, and (for some readers, at least) the "hardness" or rigor of the science itself. One requirement for hard SF is procedural or intentional: a story should try to be accurate, logical, credible and rigorous in its use of current scientific and technical knowledge about which technology, phenomena, scenarios and situations that are practically and/or theoretically possible.
Some authors scrupulously avoid such technology as faster-than-light travel, while others accept such notions (sometimes referred to as "enabling devices", since they allow the story to take place) but focus on realistically depicting the worlds that such a technology might make possible. In this view, a story's scientific "hardness" is less a matter of the absolute accuracy of the science content than of the rigor and consistency with which the various ideas and possibilities are worked out.