noun
icosahedron
A polyhedron with twenty faces. Used in geometry and related fields.
正二十面体は、20枚の正三角形で構成される。
A regular icosahedron is composed of 20 equilateral triangles.
Specifically a regular icosahedron, where all faces are congruent equilateral triangles. 二十面体 is the broader term for any 20-faced polyhedron.
Compound of 二十 (にじゅう, 'twenty'), 面 (めん, 'face'), and 体 (たい, 'body'), literally 'twenty-faced body'. A straightforward Sino-Japanese mathematical term.