Definition of si

• 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); "Today the United States is the only country in the world not totally committed to the Systeme International dUnites"
• a tetravalent nonmetallic element; next to oxygen it is the most abundant element in the earths crust; occurs in clay and feldspar and granite and quartz and sand; used as a semiconductor in transistors
• the syllable naming the seventh (subtonic) note of any musical scale in solmization