Si or si or SI may stand for:
- 'SI', the Système International d'Unités (the 'metric' International System of Units)
- one of two Spanish words pronounced like "see":
- sí (accented) for "yes" and
- si (unaccented) for "if"
- in music, Si, the seventh note of solfege
- in chemistry, Si, the chemical symbol for silicon
- in Mohican mythology, Si, the lunar deity
- Sinhalese language, (ISO 639 alpha-2)
- Slovenia (ISO 3166-1 alpha-2 country code, SI)
- in UK law, a statutory instrument
- Smithsonian Institution
- Socialist International
- Sports Illustrated
- Sports Interactive
- Situationist International
- Staten Island, a borough of New York City and an island in the Hudson River
- Swedish Institute (Svenska institutet)
- Supplemental Instruction
- Sexual Intercourse (medical abbreviation, not common usage)
- Self-injury or self-harm
- SI is the acronym for Swarm Intelligence
- In Mathematics si or Si is the Sine integral
- the IATA code for Skynet Airlines
- The Si is a self-duplicating artifact in ADOM
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