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WordNet (r) 3.0 (2006) (WordNet)

Si
n 1: 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 [synonym: {silicon}, {Si}, {atomic
number 14}]
2: 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 d'Unites" [synonym: {Systeme International
d'Unites}, {Systeme International}, {SI system}, {SI}, {SI
unit}, {International System of Units}, {International
System}]
3: the syllable naming the seventh (subtonic) note of any
musical scale in solmization [synonym: {ti}, {te}, {si}]