Definitions
from The American Heritage® Dictionary of the English Language, 5th Edition.
 noun A polyhedron having 20 faces.
from The Century Dictionary.
 noun A solid bounded by twenty planes.
from the GNU version of the Collaborative International Dictionary of English.
 noun (Geom.) A solid bounded by twenty sides or faces.
 noun one of the five regular polyhedrons, bounded by twenty equilateral triangules. Five triangles meet to form each solid angle of the polyhedron.
from Wiktionary, Creative Commons Attribution/ShareAlike License.
 noun geometry A
polyhedron withtwenty faces .  noun geometry, specifically A
regular icosahedron: one of thePlatonic solids , all of whose faces are regular triangles
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 noun any polyhedron having twenty plane faces
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
from Wiktionary, Creative Commons Attribution/ShareAlike License
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Examples

The icosahedron is another of the five regular, or Platonic, bodies having all their sides, angles, and planes similar and equal.
Amusements in Mathematics Henry Ernest Dudeney 1893

Then, about 10 years ago, the mathematician Reidun Twarock developed a more general theory of the geometry of viruses based on the symmetries of the icosahedron, but using shapes in six dimensions, not three.
New Angles on Biology Keith Devlin 2011

I've rolled the icosahedron and painted a few miniatures myself so it only raises my charisma to be accepted by that culture.
Archive 20090801 2009

I've rolled the icosahedron and painted a few miniatures myself so it only raises my charisma to be accepted by that culture.

The company receives more than 1 million applications a year and identifies the top candidates through a rigorous screening process that analyzes SAT scores, gradepoint averages and their performance on tests with such questions as: "How many different ways can you color an icosahedron with one of three colors on each face?"

The company receives more than 1 million applications a year and identifies the top candidates through a rigorous screening process that analyzes SAT scores, gradepoint averages and their performance on tests with such questions as: "How many different ways can you color an icosahedron with one of three colors on each face?"

Then, about 10 years ago, the mathematician Reidun Twarock developed a more general theory of the geometry of viruses based on the symmetries of the icosahedron, but using shapes in six dimensions, not three.
New Angles on Biology Keith Devlin 2011

The company receives more than 1 million applications a year and identifies the top candidates through a rigorous screening process that analyzes SAT scores, gradepoint averages and their performance on tests with such questions as: "How many different ways can you color an icosahedron with one of three colors on each face?"

Join five equilateral triangles together and you have the first part of an icosahedron C.
HERE’S LOOKING AT EUCLID Alex Bellos 2010

Only five shapes fit the bill: the tetrahedron, the cube, the octahedron, the icosahedron and the dodecahedron, the quintet known as the Platonic solids since Plato wrote about them in the Timaeus.
HERE’S LOOKING AT EUCLID Alex Bellos 2010
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