means A solid three-dimensional figure bounded entirely by flat polygonal faces meeting at straight edges and sharp corners, like a cube or pyramid.

from Straight from Greek: poly- ('many') joined to hedra ('seat' or 'base'). So a polyhedron is literally a 'many-seated' thing — each flat face a seat for the shape to rest on. That same hedra sits quietly inside cathedra (a bishop's chair, ancestor of 'cathedral') and chair-words across the family.