equilateral

adjective

equi·​lat·​er·​al ˌē-kwə-ˈla-tə-rəl 

ˌe-,

-ˈla-trəl

1

: having all sides equal

an equilateral triangle

an equilateral polygon

see triangle illustration

2

: having all the faces equal

an equilateral polyhedron

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

Each Magna-Tile is a translucent geometric shape, and this set includes squares (four large, 50 small) and triangles (11 right, 20 equilateral, and 15 isosceles). —Katrina Cossey, Parents, 12 July 2023

This substantial set comes with 100 pieces, including squares (large and small) and triangles (equilateral, right and isosceles). —Jessica Hartshorn, Good Housekeeping, 19 Dec. 2022

Waldo has floppy ears shaped like perfect equilateral triangles and feet too big for his lanky body, which continues to grow. —Gemma Tarlach, Discover Magazine, 25 Nov. 2014

The set includes 48 regular squares, eight large squares, eight thick-sided squares, eight squares that look like windowpanes, 16 small and 16 large equilateral triangles plus 16 isosceles triangles. —Jessica Hartshorn, Good Housekeeping, 19 Dec. 2022

Word History

Etymology

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

First Known Use

1570, in the meaning defined at sense 1

Phrases Containing equilateral

• equilateral hyperbola
• equilateral triangle

Kids Definition

equilateral

adjective

equi·​lat·​er·​al ˌē-kwə-ˈlat-ə-rəl 

ˌek-wə-,

-ˈla-trəl

: having all sides or faces equal

an equilateral triangle