1. Nomenclature (or the words they use)

Whether SF is called scientifiction, science fiction, space fiction, science fantasy or speculative fiction, it is still a literature of the future. So many different sorts of stories are classified as SF that it is hard to word a definition that will encompass all.

An inherent element in SF is that of speculation — hence the preference of some for the 'speculative fiction' title. This speculation can be of any kind. Today's SF is a far cry from the original hard science stories of the 30's and 40's.

SF writers can speculate on the consequences of many new concepts, whether of the physical sciences, social sciences or the current amalgam sciences. A.E. van Vogt created the science of 'nexialism' in The voyage of the Space Beagle and this amalgam science involved the co-ordination of all the other specialist subjects on the spaceship. Needless to say the sole nexialist aboard ends up running the whole show.

The other element in all SF (or should be) is the fiction element. SF should tell a story. Whether there is a message or vision behind the story or not doesn't matter The story is there to be told and the reader may or may not get anything extra from the story. Of course, some stories are better than others, and a message or a vision can improve a story.

Science is a very grey element in a lot of current SF. The early stories all dealt with scientific extrapolation, but as science has expanded, so too has the range of materials used in SF. Depending on you definition, science is still important in SF, but the sort of science is changing.