An Etymological Dictionary of Astronomy and Astrophysics

The branch of mathematics dealing with the relations of the sides and angles of triangles; also the various algebraic functions of these relations.

Etymology (EN): From N.L. trigonometria, from Gk. trigonon “triangle” (from tri-, → three, + gonia “angle,” cognate with Pers. zânu “knee”) + metron,
→ -metry.

Etymology (PE): Sebarsanji, from sebar, → trianle, + -sanji, → -metry.