icosahedron (n.)
"twenty-sided body," 1560s, from Latinized form of Greek eikosahedron, noun use of neuter of eikosahedros, from eikosi "twenty" + -hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit." Greek eikosi is from PIE *wikmti- "twenty," from *wi- "in half" (hence "two") + (d)kmti-, from root *dekm- "ten" (from PIE root *dekm- "ten"). Related: icosahedral.