Pronunciation: (si-lē'nē-um),[key] —n.Chem. a nonmetallic element chemically resembling sulfur and tellurium, occurring in several allotropic forms, as crystalline and amorphous, and having an electrical resistance that varies under the influence of light. Symbol: Se; at. wt.: 78.96; at. no.: 34; sp. gr.: (gray) 4.80 at 25°C, (red) 4.50 at 25°C.