Trigonometry is a branch of mathematics that studies relationships between side lengths and angles of triangles. The field emerged in the Hellenistic world during the 3rd century BC from applications of geometry to astronomical studies. The Greeks focused on the calculation of chords, while mathematicians in India created the earliest-known tables of values for trigonometric ratios (also called trigonometric functions) such as sine, cosine, and tangent.
What is a chord?
The word chord refers to a straight line drawn between two points
on a circle (or more generally, on any curve). The known first
trigonometric table was a table of chords. In modern times, the
sine is used instead (sines and chords are closely related), but,
perhaps, chords are more intuitive.
For example, the angle CAD in the diagram shows the curve
connecting C to D to be an arc of a circle. The straight line CD is
the chord. Of course, the length of the chord depends on the radius of
the circle, in fact, it is proportional to the radius of the circle.