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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Did You Know?

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 Its detectors will be arranged in an equilateral triangle with sides 2.5m kilometres long. The Economist, "Gravitational astronomy proves its maturity," 22 Aug. 2019 The nucleus’s surplus energy enables its alpha clusters to stretch farther apart from one another than the clusters in ground-state carbon-12, which draw together in a tight equilateral triangle. Quanta Magazine, "A Primordial Nucleus Behind the Elements of Life," 4 Dec. 2012 Cobb’s response was to create a building on which every floor plan above the fifth level has the shape of an equilateral triangle. Robert Campbell, BostonGlobe.com, "One Dalton, Boston’s commanding new skyscraper, conjures architectural magic," 29 June 2019 Imagine the floor of your bedroom is tiled with regular hexagons, all the same size, and each hexagon is divided into six equilateral triangles. Quanta Magazine, "The Brain Maps Out Ideas and Memories Like Spaces," 14 Jan. 2019 In the two-dimensional plane, there’s a candidate for universal optimality — the equilateral triangle lattice — but no proof. Quanta Magazine, "Out of a Magic Math Function, One Solution to Rule Them All," 13 May 2019 Select three colors that draw an equilateral triangle on the wheel; royal purple, orange, and green keep especially good company. Mike Irvine, Sunset, "4 Ways to Design Colorful Floral Arrangements," 22 Jan. 2018 The concept for the Einstein Telescope in Europe calls for a detector with 10-kilometre arms arranged in an equilateral triangle and placed in tunnels 100 metres or so underground. Davide Castelvecchi, Scientific American, "Here Come the Waves," 23 Apr. 2018 Soon after sunset on August 11, turn your attention to the gibbous moon forming an eye-catching, equilateral triangle pattern with the brilliant planets Mars and Saturn. National Geographic, "This Week’s Night Sky: See an Asteroid Pass and Meteors Peak," 8 Aug. 2016

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.

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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

Time Traveler

The first known use of equilateral was in 1570

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Cite this Entry

“Equilateral.” The Merriam-Webster.com Dictionary, Merriam-Webster Inc., https://www.merriam-webster.com/dictionary/equilateral. Accessed 7 December 2019.

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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

Spanish Central: Translation of equilateral

Nglish: Translation of equilateral for Spanish Speakers

Comments on equilateral

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