The Collaborative International Dictionary of English v.0.48:

science fiction \sci"ence fic"tion\ (s[imac]"ens
   f[i^]k"sh[u^]n), n. [science fiction.]
   A genre of fiction in which scientific and technological
   issues feature prominently, especially including scenarios in
   which speculative but unproven scientific advances are
   accepted as fact, and usually set at some time in the future,
   or in some distant region of the universe.
   [PJC]

WordNet (r) 3.0 (2006):

science fiction
    n 1: literary fantasy involving the imagined impact of science
         on society

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "science fiction":
   Marchen, Western, Western story, Westerner, adventure story,
   allegory, apologue, bedtime story, detective story, fable, fabliau,
   fairy tale, fantasy, fiction, folk story, folktale, gest,
   ghost story, horse opera, legend, love story, mystery,
   mystery story, myth, mythology, mythos, nursery tale, parable,
   romance, shocker, space fiction, space opera, suspense story,
   thriller, whodunit, work of fiction