"Hard SF" is a sub-genre of Science Fiction where "Science" is key. It's very hard to draw solid lines around stories that are examples of Hard SF, but it's easier to provide examples of things which are not: "Star Wars", for example, where space ships, travel and light sabres are window-dressing around a fantasy story. If you look at the Wikipedia article on Hard SF, in particular it's edit history, you will see people continually argue back and forth. Editor Jonathan Strahan recently asked for some recommendations of Hard SF authors to be included in a new Hard SF anthology. I stumbled across this via sfsignal. There are some good comments at both sites, and a fair amount of disagreement over whether certain authors should be considered Hard SF authors or not. Alastair Reynolds was cited by people several times. I really enjoy much of Reynold's writing, but I wouldn't consider it Hard SF. I argued this point and Reynolds himself joined the discussion.
This page is a draft.