equi·​lat·​er·​al | \ ˌē-kwə-ˈla-tə-rəl How to pronounce equilateral (audio) , ˌe-, -ˈla-trəl \

Definition of equilateral

1 : having all sides equal an equilateral triangle an equilateral polygon — see triangle illustration
2 : having all the faces equal an equilateral polyhedron

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Since equi- means "equal", the meaning of equilateral is easy to guess from its roots. The word is mostly used in geometry. The standard polygons (many-sided geometrical shapes)—the pentagon, hexagon, octagon, etc.—are assumed to be equilateral if we don't say otherwise; an equilateral rectangle has the special name square. But triangles are particularly important, and many triangles are not equal-sided. The standard polyhedrons (many-sided solids) are also equilateral. Most common is the cube, all of whose sides are square. The tetrahedron has four triangular sides and thus is a pyramid with a triangular base, unlike the pyramids of Egypt with their square bases.

Examples of equilateral in a Sentence

Recent Examples on the Web Three Lagrange points are unstable and lie along that line (L1, L2, and L3), and two are stable (L4 and L5) and symmetric, above and below that line as points of an equilateral triangle. Manasee Wagh, Popular Mechanics, 29 Apr. 2022 His idealized male figure features legs spread wide to form an equilateral triangle. Los Angeles Times, 12 Nov. 2021 Three tethered pit bulls — big, horizontal heads and huge jaws with those broad, goofy smiles — are in one large yard, 60 feet apart from each other in a near equilateral triangle, not able to touch or play. Gene Weingarten, Washington Post, 8 Nov. 2021 Achieving balance with the three M’s in the context of business outcomes should look like an equilateral pyramid where three M’s form the base and business outcome serve as the apex and define healthy or balanced ITOps 4.0. Murthy Malapaka, Forbes, 12 May 2021 As early as the 1400s scientists began to suspect that the regular tetrahedron — in which all four faces of the pyramid are equilateral triangles — can’t be used to fill space either. Quanta Magazine, 9 Feb. 2021 The group can be expressed as a set of mathematical matrices — arrays of numbers that, when multiplied by coordinates of an equilateral triangle, return the same coordinates. Quanta Magazine, 12 Nov. 2020 One popular design is based on the mathematical structure of an icosahedron, a three-dimensional shape built from equilateral triangles. Eugenia Cheng, WSJ, 19 Nov. 2020 For the four Platonic solids built out of squares or equilateral triangles — the cube, tetrahedron, octahedron and icosahedron — mathematicians recently figured out that the answer is no. Quanta Magazine, 31 Aug. 2020 See More

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'equilateral.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

First Known Use of equilateral

1570, in the meaning defined at sense 1

History and Etymology for equilateral

Late Latin aequilateralis, from Latin aequi- + later-, latus side — more at lateral

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Time Traveler for equilateral

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The first known use of equilateral was in 1570

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equilateral arch

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Last Updated

6 May 2022

Cite this Entry

“Equilateral.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/equilateral. Accessed 28 Jun. 2022.

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More Definitions for equilateral


equi·​lat·​er·​al | \ ˌē-kwə-ˈla-tə-rəl How to pronounce equilateral (audio) , ˌe-kwə- \

Kids Definition of equilateral

: having all sides or faces equal an equilateral triangle

More from Merriam-Webster on equilateral

Nglish: Translation of equilateral for Spanish Speakers


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