正せい八はち面めん体たい

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Meanings

1

noun

regular octahedron

A polyhedron with eight congruent equilateral triangular faces; one of the five Platonic solids.

正correct, justice, righteous

八eight, eight radical (no. 12)

面mask, face, features

体body, substance, object

正せい多た面めん体たい
正多面体 is the general term for regular polyhedra; 正八面体 is a specific type.
正せい四し面めん体たい
正四面体 is a regular tetrahedron (4 faces), while 正八面体 has 8 faces.

Compound of 正 (regular) + 八面体 (octahedron). 八面体 itself is from 八 (eight) + 面 (face) + 体 (body).