The fact is, "sci-fi" was long treated as a disparaging term by science fiction authors, because of the way the mass media used it disparagingly in the '60s-'80s or so. Many SF authors and fans perceived it as an insult, and many probably still do. It came to be associated with the more superficial, lowbrow stuff that generally passes for SF in the mass media. Today, it probably has less of a stigma attached than it used to, because SF has become more respectable in the mass media. But I consider it to be a useful term for the mass-media variant of science fiction/fantasy/adventure/what-have-you, the generally "soft"-science stuff that isn't as conceptually deep, tends to blur genre lines, and focuses more on visual spectacle. The simple truth is that the mass-media take on the genre is very different from the prose version, so it's useful to use "sci-fi" as a distinguishing label for the former.
However, it is grossly incorrect to say that science fiction needs to be scientifically accurate. That excludes a wealth of respected SF by authors such as Ray Bradbury, Richard Matheson, Ursula LeGuin, etc. Hard science fiction is the subset of the genre that's grounded in plausible science, but the term "science fiction" as a whole is far more encompassing than that.
To make an attempt at defining the two terms usefully: Science fiction is a genre of fiction based on postulating hypothetical new scientific discoveries, technological or sociological innovations, or other changes to the world and exploring their ramifications in a thoughtful way. Sci-fi is a genre or style of fiction that uses the trappings and syntax of science fiction, often blended indiscriminately with elements from fantasy, horror, and other genres, to tell stories that are generally more conventional action-adventure, character-drama, or humor narratives.
However, it is grossly incorrect to say that science fiction needs to be scientifically accurate. That excludes a wealth of respected SF by authors such as Ray Bradbury, Richard Matheson, Ursula LeGuin, etc. Hard science fiction is the subset of the genre that's grounded in plausible science, but the term "science fiction" as a whole is far more encompassing than that.
To make an attempt at defining the two terms usefully: Science fiction is a genre of fiction based on postulating hypothetical new scientific discoveries, technological or sociological innovations, or other changes to the world and exploring their ramifications in a thoughtful way. Sci-fi is a genre or style of fiction that uses the trappings and syntax of science fiction, often blended indiscriminately with elements from fantasy, horror, and other genres, to tell stories that are generally more conventional action-adventure, character-drama, or humor narratives.