rhombohedron
Concise Oxford English Dictionary © 2008 Oxford University Press:
rhombohedron /ˌrɒmbə(ʊ)ˈhiːdr(ə)n, -ˈhɛd-/
▶noun (pl. rhombohedra /-drə/ or rhombohedrons) a solid figure whose faces are six equal rhombuses.
– derivatives
rhombohedral adjective.
rhombohedral adjective.
'rhombohedron' also found in these Oxford entries:

