noun

• Initialism of South Dakota. [A state in the Upper Midwest region of the United States. Capital: Pierre. Largest city: Sioux Falls.]
• (uncountable) Initialism of Secure Digital: a non-volatile memory card format developed for use in portable devices.
• Initialism of school district. [(Canada, US) An administrative unit responsible for public primary and secondary schools in a particular locality; the area under the jurisdiction of such an administrative unit.]
• (law enforcement) Initialism of sheriff's department.
• (mathematics) Initialism of standard deviation. [(statistics) A measure of how spread out data values are around the mean, defined as the square root of the variance. Represented with the Greek letter σ (sigma).]
• (video games) Initialism of self-destruct.
• (genetics) Initialism of segmental duplication.
• (television) Initialism of standard definition (“typically 640×480 pixels”). [(television) A system that uses a resolution that is not considered to be either high-definition television (720p, 1080i, 1080p, 1440p, 4K UHDTV, and 8K UHD) or enhanced-definition television (EDTV 480p).]
• Alternative letter-case form of SD (“standard deviation”)