正せい四し面めん体たい

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Meanings

1

noun

regular tetrahedron

A polyhedron with four congruent equilateral triangular faces; a fundamental shape in geometry.

correct, justice, righteous

四four

面mask, face, features

体body, substance, object

四し面めん体たい
四面体 is a general tetrahedron, not necessarily regular. 正四面体 specifically has all edges equal and faces congruent.
正せい多た面めん体たい
正多面体 is the broader category of regular polyhedra; 正四面体 is one of the five Platonic solids.

Compound of 正 (regular) + 四面体 (tetrahedron). The term follows standard mathematical naming conventions.