# Definition:Icosahedron

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## Contents

## Definition

An **icosahedron** is a polyhedron which has $20$ faces.

### Regular Icosahedron

A **regular icosahedron** is an icosahedron whose $20$ faces are all congruent equilateral triangles.

### Truncated Icosahedron

A **truncated icosahedron** is an icosahedron whose $12$ vertices are truncated such that one third of each edge is cut off at each of both ends.

## Also see

- Results about
**icosahedra**can be found here.

## Linguistic Note

The word **icosahedron** derives from the Classical Greek **εἰκοσάεδρον**:

**eíkosi**(**εἴκοσι**), meaning**twenty****hedron**(a form of**ἕδρα**), meaning**base**or**seat**.

The technically correct plural of **icosahedron** is **icosahedra**, but the word **icosahedrons** can often be found.

## Sources

- 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $20$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $20$ - 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next): Entry:**icosahedron**(*plural***icosahedra**) - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next): Entry:**icosahedron**(*plural***icosahedra**) - 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next): Entry:**icosahedron (icosahedra)**