Si

Si, si, or SI may stand for:
*The International System of Units, the modern international standard version of the metric system; SI being the abbrieviation of its French title, Système International d'Unités
*Si is the chemical symbol for silicon
*Seletar Institute, a defunct centralised institute in Singapore; one of the original 4 CIs.
*one of two Spanish words pronounced like "see":
*sí (accented) for "yes" and
*si (unaccented) for "if"
*a French word pronounced like "see" meaning "yes, on the contrary", "as," "however," "so," and "if."
*in music, Si, the original name of the seventh note of solfege; it was later changed to "Ti"
*in Mohican mythology, Si, the lunar deity
*Station Inspector the highest non-commissioned officer rank in the Singapore Police Force.
*Sinhalese language, (ISO 639 alpha-2)
*Slovenia (ISO 3166-1 alpha-2 country code, SI)
*in British law, a Statutory Instrument
*Smithsonian Institution
*Socialist International
*Spokane International Railroad
Sports Illustrated
*Sports Interactive
*Signal Integrity
*Situationist International
*Staten Island, a borough of New York City and an island in the Hudson River
*Swedish Institute (Svenska institutet)
*System integrator
*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-replicating artifact in the roguelike game Ancient Domains of Mystery
*The Si designation is used by Honda to denote high-performance variants of models they manufacture
*The Shift in control code in the C0 control code set.
*System Information, a type of packet in a MPEG-2/DVB stream.
*Scouting Ireland