equilateral

adjective equi·lat·er·al \ ˌē-kwə-ˈla-tə-rəl , ˌe- , -ˈla-trəl \

|Updated on: 2 May 2018

Editor's Note: Did You Know?

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

See equilateral defined for kids

Recent Examples of equilateral from the Web

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
In the top row of the tablet, said Wildberger, relatively equal ratios create a near equilateral triangle.

— sarah gibbens, National Geographic, "Ancient Tablet May Show Earliest Use of This Advanced Math," 25 Aug. 2017
The icosahedron is made from 20 identical equilateral triangles and seemed to exist nowhere except in mathematicians’ imaginations.

— eugenia cheng, WSJ, "Lessons of the Möbius Bagel," 10 Aug. 2017
The new metropolis would consist of a giant tetrahedron—a pyramid made of equilateral triangles—a shape that Fuller, the popularizer of the geodesic dome, admired for its stability and symmetry.

— rachel riederer, New Republic, "Libertarians Seek a Home on the High Seas," 29 May 2017
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.

— andrew fazekas, 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.

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.