octahedron

Concise Oxford English Dictionary © 2008 Oxford University Press:
octahedron /ˌɒktəˈhiːdrən, -ˈhɛd-/

▶noun (pl. octahedra /-drə/ or octahedrons) a three-dimensional shape having eight plane faces, in particular a regular solid figure with eight equal triangular faces.

– derivatives
octahedral adjective.

– origin C16: from Gk oktaedron, neut. (used as n.) of oktaedros ‘eight-faced’.