Plural of enneahedron; multiple solid three-dimensional shapes with nine faces.
From Greek 'ennea' (nine) + 'hedra' (face), with '-a' as the Greek plural ending. In geometry, '-hedra' specifically refers to the flat surfaces of a polyhedron.
While most people know cubes (6 faces) and pyramids, nine-faced solids ('enneahedra') are far less famous even though they appear in crystal structures and are studied in advanced mathematics—it's a reminder that geometry goes much deeper than what's taught in basic classes.
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