Meaning

noun 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 transist

noun 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)

noun the syllable naming the seventh (subtonic) note of any musical scale in solmization

originFrom Middle English si (“seventh degree or note of Guido of Arezzo's hexachordal scales”), Italian si in the solmization of Guido of Arezzo, from the initials of Latin Sāncte Iohannēs (“Saint John (the Baptist)”) in the lyrics of the scale-ascending hymn Ut queant laxis by Paulus Deacon; thus, also an initialism of Sāncte Iohannēs.

Etymology from Wiktionary, CC BY-SA 3.0