Trigonometry emerged in the Hellenistic world during the 3rd century BCE from applications of geometry to astronomy; the Greeks focused on the calculation of chords, while mathematicians in India created the earliest known tables of values for trigonometric functions such as sine.
1892, Edward Albert Bowser, A Treatise on Plane and Spherical Trigonometry, D. C. Heath & Co., page 1,
Trigonometry was originally the science which treated only of the sides and angles of plane and spherical triangles; but it has been recently extended so as to include the analytic treatment of all theorems involving the consideration of angular magnitudes.
2013, Paul Abbott, Hugh Neill, Trigonometry: A Complete Introduction, Hachette, unnumbered page,
In fact, the earliest practical uses of trigonometry were in the fields of astronomy and hence navigation.
2016, Carl F. Lorenzo, Tom T. Hartley, The Fractional Trigonometry, Wiley, page 8,
The properties of these new trigonometries and identities flowing from the definitions are then developed.
The trigonometries derived from these generalizations will be jointly termed "The Fractional Trigonometry."
(branch of mathematics):trig(informal, abbreviation)