-hedroncomb. form

Etymology: < post-classical Latin -hedron, -hedrum, -edrum and its etymon ancient Greek -εδρον (in e.g. ὀκτάεδρον octahedron n.), neuter singular of Hellenistic Greek -εδρος (in e.g. ὀκτάεδρος ), forming adjectives designating mathematical figures < ancient Greek ἕδρα seat < the same base as ἕδος seat, ἕζεσθαι to sit (see sit v.). Compare French -edre.The -h- , not present in the ancient Greek forms (e.g. ὀκτάεδρον ) already occurs, after the Greek simplex, in post-classical Latin, e.g. octaedrum , octahedrum octahedron n. Earliest in a number of borrowings and adaptations of Greek and Latin words in the second half of the 16th cent., e.g. octahedron n., polyhedron n., pentahedron n. N.E.D. gives also the pronunciation (-he·drǫ̆n) /-ˈhɛdrən/ for many words showing this element.