Solomon Islands, a Pacific island nation

Staten Island, a borough of New York City and an island in the Hudson River

Si, the chemical symbol for silicon

Sports Illustrated, an American sports magazine

Système International d'Unités (International System of Units), the modern international standard version of the metric system

Sergeant Instructor, an appointment in the British Army

Sexual intercourse, a medical abbreviation

Smithsonian Institution, an American educational and research institute

Self-injury summary and related information.

Statutory Instrument, a form of delegated or secondary legislation in Great Britain

Si, the original name of the seventh note of solfege, later changed to "Ti"

Signal integrity, electronic circuit tools and techniques that ensure electrical signals are of sufficient quality for proper operation

Scouting Ireland, the national Scouting association of Ireland

Saint Ignatius College Prepatory

The Elder Scrolls IV: Shivering Isles, an expansion pack for The Elder Scrolls IV: Oblivion

Socialist International, a worldwide organisation of social democratic, labour, and democratic socialist political parties

Swarm Intelligence, an artificial intelligence technique

ISO 639 alpha-2 code for the Sinhalese language

Si, the lunar deity in Mahican mythology

si (unaccented), a pronoun

Seletar Institute, a defunct centralised institute in Singapore

Si, the person focus-marker in the Chamorro language

Si, another name for the Sídhe, Celtic mythological beings

sí (accented) for "yes" or the reflexive pronoun.

Si(x), the sinc function in mathematics

Si, a self-replicating artifact in the computer game Ancient Domains of Mystery

.si, the Internet country code top-level domain for Slovenia

Si, a shortening of the first name Simon

Samahang Ilokano, a fraternity/sorority based in the Philippines

Suicidal ideation, a medical term for thoughts about suicide

Sports Interactive, a British computer games development company

System integrator, a person or company that specialises in integrating systems

Sì (song), the name of the Italian entry to the Eurovision Song Contest 1974

si (unaccented) for "if".

Reporting mark of Spokane International Railroad, a former railway in Washington, USA

Si, the sine integral in mathematics

Sí (Julieta Venegas), an album by Mexican singer and songwriter Julieta Venegas

Survey of India, an Indian government agency responsible for managing geographical information about India

S1 ACPI S1 power state in computing S1 (military), an administrative position within military units. S1 (Berlin), a line on the Berlin S-Bahn S1 Invicta (car) In human anatomy it can refer to either: First sacral vertebra of the vertebral column In the...

IATA code for Skynet Airlines

Swedish Institute (Svenska institutet), a Swedish government agency which promotes Sweden abroad

SÍ Sørvágur, a Faroese sports association

Sensory integration, the neurological process that organizes sensation from one's own body and the environment

Station Inspector, the highest non-commissioned rank in the Singapore Police Force

Service Information, a type of metadata packet in a MPEG-2/DVB stream

Shift In, a control code in the C0 control code set