Etymology
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si

"yes" in Italian and Spanish; from Latin sic "so" (see sic).

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Definitions of si
1
si (n.)
a complete metric system of units of measurement for scientists; fundamental quantities are length (meter) and mass (kilogram) and time (second) and electric current (ampere) and temperature (kelvin) and amount of matter (mole) and luminous intensity (candela);
Synonyms: Systeme International d'Unites / Systeme International / SI system / SI unit / International System of Units / International System
si (n.)
the syllable naming the seventh (subtonic) note of any musical scale in solmization;
Synonyms: ti / te
2
Si (n.)
a tetravalent nonmetallic element; next to oxygen it is the most abundant element in the earth's crust; occurs in clay and feldspar and granite and quartz and sand; used as a semiconductor in transistors;
Synonyms: silicon / atomic number "
From wordnet.princeton.edu