trigonometry (n.) Look up trigonometry at Dictionary.com
"branch of mathematics that deals with relations between sides and angles of triangles," 1610s, from Modern Latin trigonometria (Barthelemi Pitiscus, 1595), from Greek trigonon "triangle" (from tri- "three," see tri-, + gonia "angle," see knee (n.)) + metron "a measure" (see meter (n.2)).