An Etymological Dictionary of Astronomy and Astrophysics

The plane figure formed by three lines intersecting in pairs at three points; a three-sided → polygon. → equilateral triangle, → isosceles triangle, → scalene triangle.

Etymology (EN): M.E., from O.Fr. triangle, from L. triangulum “triangle,” from neuter of adj. triangulus “three-cornered,” from tri-, → three, + angulus “corner,” → angle.

Etymology (PE): Seguš “three-cornered,” from sé, → three,

• guš “corner, → angle.”
  Sebar “three-sided,” from sé, → three,
• bar “→ side; breadth; breast.”