science fiction

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A literary or cinematic genre in which fantasy, typically based on speculative scientific discoveries or developments, environmental changes, space travel, or life on other planets, forms part of the plot or background.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Fiction in which advanced technology and/or science is a key element.
• noun Technology that, while theoretically possible, is not yet practical.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun literary fantasy involving the imagined impact of science on society