Places

• South Dakota, United States postal abbreviation
• Sudan, ISO country code
• Sudan Airways, IATA airline designator
• San Diego, California
• San Diego County, California
• San Diego, Texas

In technology

• SanDisk, US-based multinational corporation which designs and markets flash memory card products
• SafeDisc, a CD/DVD copy protection solution by Macrovision Corporation
• Secure Digital card, flash memory card format used in portable device, including digital cameras and handheld computers
• Self Destruct, in some video games.
• Software Distributor, an HP-UX software package management system
• Standard-definition television
• SharpDevelop, a free and open source IDE for the C# Visual Basic .NET programming languages

In politics

• Sicherheitsdienst, Security Police, the intelligence service of the Nazi SS
• Sinistra Democratica, Italian political party.
• Social Democrats (Slovenia), a left-wing Slovenian political party
• Stronnictwo Demokratyczne, a defunct Polish party
• Swiss Democrats (Schweizer Demokraten) A right-wing - political party in Switzerland

Silenced, put after a gun name to indicate the presence of a *Silencer*

Government

• School district

In mathematics

• Standard deviation, statistics
• Significant digits, a method of expressing error in measurement
• Systems dynamics modeling

In Medicine

• Spasmodic dysphonia
• Schizoaffective disorder

In Music

• Godin SD, a guitar manufactured by Godin Guitars, a Canadian guitar

Other

• Sales and Distribution, business
• Silver & Deming, a type of reduced-shank drill bit
• Sindhi language
• Suppressed, a suffix placed at the end of a firearm's model name to note it is fitted with a suppressor.
• Sustainable Development, Environmental, economic and social development that meets the needs of the present without compromising the needs of future generations
• Super deformed, an art style in which characters are drawn with short, stubby, highly exaggerated builds
• SmackDown!, an abbreviation for SmackDown! a WWE Friday night show
• Scooby Doo
• SD1 (further disambiguation)