Pronunciation: (sil'i-kun, -kon"),[key] —n.Chem. a nonmetallic element, having amorphous and crystalline forms, occurring in a combined state in minerals and rocks and constituting more than one fourth of the earth's crust: used in steelmaking, alloys, etc. Symbol: Si; at. wt.: 28.086; at. no.: 14; sp. gr.: 2.4 at 20°C.