Trigonometry (from Greek trigōnon, "triangle" + metron, "measure") is a branch of mathematics that studies relationships involving lengths and angles of triangles. 

The field emerged during the 3rd century BC from applications of geometry to astronomical studies. 

The 3rd-century astronomers first noted that the lengths of the sides of a right-angle triangle and the angles between those sides have fixed relationships: that is, if at least the length of one side and the value of one angle is known, then all other angles and lengths can be determined algorithmically. 

These calculations soon came to be defined as the trigonometric functions and today are pervasive in both pure and applied mathematics: fundamental methods of analysis such as:

  •  the Fourier transform, for example, or 
  • the wave equation, use trigonometric functions 
  • to understand cyclical phenomena across many applications in fields as diverse as 
    • physics, 
    • mechanical and electrical engineering, 
    • music and acoustics, 
    • astronomy, 
    • ecology, and 
    • biology. 

Trigonometry is also the foundation of the practical art of surveying.

It's mostly about right angle triangles, even if they aren't right angled

Trigonometry is most simply associated with planar right-angle triangles (each of which is a two-dimensional triangle with one angle equal to 90 degrees). 

The applicability to non-right-angle triangles exists, but, since any non-right-angle triangle (on a flat plane) can be bisected to create two right-angle triangles, most problems can be reduced to calculations on right-angle triangles. 

Thus the majority of applications relate to right-angle triangles. One exception to this is spherical trigonometry, the study of triangles on spheres, surfaces of constant positive curvature, in elliptic geometry (a fundamental part of astronomy and navigation). Trigonometry on surfaces of negative curvature is part of hyperbolic geometry.

Trigonometry basics are often taught in school, either as a separate course or as a part of a precalculus course.