In the 1966 Encyclopedia Britannica, “science fiction” was defined as having two basic types. The first is science fiction proper, “an almost step-by-step development of possibilities from known scientific or social data.” 1984 was given as an example. The other type is science fantasy, which “can leap directly to whatever farfetched requirement of dramatic plausibility” and “permits the imagination not only to go beyond the known and proved but to contradict it when necessary”. Ray Bradbury’s The Martian Chronicles is mentioned as an example. The article goes on to mention that “the two [types] are often combined”.
In 1986, the Encyclopedia Britannica (macromedia) goes into less detail, mentioning the origin of science fiction and defining science fiction as a genre “in which the fiction writer treats how scientific discoveries, technological developments, and future events and societal changes affect human beings. The description of these influences may be a careful and informed extrapolation of scientific facts and principles, or it may range into farfetched areas flatly contradictory of such facts and principles.”
Wikipedia (~2006) is a different case entirely. It plainly starts out not by defining science fiction, but rather saying how sci-fi differs from fantasy, in that “its imaginary elements are largely possible within scientifically established or scientifically postulated laws of nature (though some elements in a story might still be pure imaginative speculation).”
There are, of course, points in common for all three. What's interesting is which of these are shared. For instance, all point to dystopian, apocalyptic and alternative histories as science fiction, three fields which are typically separated from sci-fi. All three list 1984 as science fiction. All three allow some fantastic (from fantasy) leeway. Obviously, each has its own differentials, not including the fact that science fiction developed mostly throughout the 20th century and so it’s quite logical that the articles would be quite different from each other. It's that the similarities are mostly things people assume to be outside the realm of science fiction that has my eyebrows raised.
It's that even Wikipedia sticks to this definition that is most surprising. Not because I think it's an inaccurate way to look at the genre (quite the contrary, actually), but rather that Wikipedia tends to reflect the views of the crowd. And while I may be wrong, it's always appeared to me like the crowd (readers and writers alike) wants to keep dystopian literature far from the phrase "sci-fi" (look at Margaret Atwood and "speculative" - link contains spoilers beyond the directed paragraph). There's something curiously great to the fact that this one key part to sci-fi's definition has remained the same throughout the years. Also interesting to note is the slow move to gender equality in the articles. 1966 has no mention of women sci-fi writers whatsoever. 1986 mentions Frankenstein as the "first" sci-fi book, and Ursula Le Guin among the listed authors. 2006 (Wikipedia) does better in that it provides the king of sci-fi author lists, but it actually disappoints a little too - I'd have expected a packed "Women in Science Fiction" article. No such luck.