A flat shape with three straight sides and three corners, or an instrument or object shaped like this.
From Latin *triangulum* “three‑cornered,” from *tri-* “three” + *angulus* “angle, corner.” It entered English through Old French in the Middle Ages, first in geometry and music.
Triangles are the simplest shape that can’t be bent without changing side lengths, which is why engineers love them for building bridges and towers. Even a flimsy metal triangle in an orchestra relies on that same rigid geometry to ring clearly.
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