A solid figure with twenty plane faces, especially equilateral triangular ones.
- ‘Hypsicles relates that his father and Basilides studied a treatise by Apollonius on a dodecahedron and an icosahedron in the same sphere and decided that Apollonius's treatment was not satisfactory.’
- ‘Four more Archimedeans appear in the Short book on the five regular solids: the truncated cube, the truncated octahedron, the truncated icosahedron and the truncated dodecahedron.’
- ‘In particular he identified the five elements, fire, earth, air, water and celestial matter with the five regular solids, the tetrahedron, cube, octahedron, icosahedron and the dodecahedron.’
- ‘One of Plato's perfect solids, the icosahedron has 20 triangular faces.’
- ‘Water, because it is the most mobile and fluid, has to be an icosahedron, the regular solid that rolls most easily.’
Late 16th century: via late Latin from Greek eikosaedron, neuter (used as a noun) of eikosaedros ‘twenty-faced’.
We take a look at several popular, though confusing, punctuation marks.