is a two-letter abbreviation which may refer to:
- San Francisco, California, a city with a population of more than 730,000, located on the West Coast of the United States.
- Sioux Falls, South Dakota, a city with a population of over 140,000, the center of the Midwest.
- Sofia, Bulgaria, the capital city of Bulgaria
- Speculative fiction, term coined by Robert A. Heinlein as a synonym for science fiction, but now refers to a literary genre which includes science fiction. Also abbreviated "spec-fic," "S-F", or "sf".
- Science Fiction, Literary genre, also abbreviated as "Sci-Fi".
- Scuderia Ferrari, the racing division of the Ferrari automobile company
- Significant figures (in mathematics)
- Spontaneous fission, a nuclear decay route in which a superheavy nucleus splits into two.
- SourceForge, a collaborative software development management system
- Special forces, small, highly trained and specialised military units
- Security Forces, US Air Force's version of MP and ground infantry
- Stade Français Paris, a French rugby union team
- Statsforetak, a type of Norwegian state enterprise
- Six Flags, a chain of theme parks
- Svensk Filmindustri, the Swedish Film Industry
- Tassili Airlines, IATA airline designator
- Salad Fingers, a flash animation created by David Firth
- Salmon farming, farming of salmon (see Fish farming)
- Selle Français, French breed of horse
- Superleague Formula, a new motor racing formula mixing football (soccer)and motorsports
In video games:
In defunct uses: