What does equilateral mean?
"Equilateral" is an adjective from mathematics describing a shape — most famously a triangle — in which all sides are of exactly equal length. It comes from the Latin roots aequus, meaning "equal," and latus, meaning "side," so it literally means "equal-sided." The word belongs to formal and technical registers: you will meet it in geometry lessons, architectural plans, crystallography, and design writing rather than casual conversation. Its most familiar collocation by far is "equilateral triangle," whose three equal sides also force its three angles to be identical at sixty degrees each — a fact that makes equilateral figures central to symmetry, tessellation, and classical construction problems solvable with compass and straightedge. A useful distinction is with "equiangular," which describes equal angles rather than equal sides; the two coincide in triangles but not in every polygon. For anyone describing balance, uniformity, or geometric regularity precisely, "equilateral" offers a compact, exact term that plainer language cannot match.
adjectiveHaving all sides of equal length; especially used of a polygon or triangle whose sides are all equal.
- Having all sides of equal length (chiefly of triangles and other polygons).
- Rarely as a noun: an equilateral figure or side, chiefly in older or technical mathematical writing.
"The children learned to construct an equilateral triangle using only a compass and straightedge."
"Each face of the die was cut from an equilateral triangle of polished hardwood."
"The tiling pattern relied on rows of interlocking equilateral hexagons."
Every equilateral triangle hides a neat secret: its angles are all exactly 60 degrees — no exceptions, no matter how big or small it is.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of equilateral
"Equilateral" derives from Latin, combining aequus ("equal") with lateralis ("of the side"), from latus ("side"). It passed into English in the sixteenth or seventeenth century through scholarly and mathematical Latin. It shares its first root with words such as "equator," "equity," and "equilibrium," and its second root with "lateral," "latitude," and "collateral."
How equilateral is actually used
"Equilateral" is a formal, technical term found almost exclusively in mathematics, geometry teaching, design, and scientific writing; it is rarely used in everyday speech outside fixed phrases like "equilateral triangle."
Easily confused with equilateral
Equiangular means having all angles equal, whereas equilateral means having all sides equal; in triangles the two coincide, but in polygons such as rectangles they do not.
An isosceles triangle has only two equal sides, while an equilateral triangle has three.