Trigonometry is a branch of mathematics which deals with triangles, particularly triangles in a plane where one angle of the triangle is 90 degrees (right triangles).
Triangles on a sphere are also studied, in spherical trigonometry.
Trigonometry specifically deals with the relationships between the sides and the angles of triangles, that is, the trigonometric functions, and with calculations based on these functions.
Trigonometry has important applications in many branches of pure mathematics as well as of applied mathematics and, consequently, much of science and precalculus.
For example, the robotic arm on the International Space Station is operated by controlling the angles of its joints.
Calculating the final position of the astronaut at the end of the arm requires repeated use of the trigonometric functions of those angles.