It is a relationship where one input results in many outputs. A common example is square roots.
the square root of 4 is -2 as well as +2. In fact, all positive numbers have two square roots: one negative and one positive. So that is an example of a one-to-many relation.
Mathematically, such a relation is not a function. However, by restricting the codomain (range) to only non-negative (or only non-positive) values the relation can be made into a function.
Similarly, the inverse functions for all six trigonometric ratios must have restricted codomains. Otherwise, because of their periodicity, each input has infinitely many outputs.
For example, arctan[sqrt(3)] = pi/3 + k*pi = pi*(1/3+k) radians, where k is any integer.
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Lemma
Pythagoras education was mathematics and was taught by other people
In geometry, angles are studied mostly in relation to each other. In Trigonometry, angles are studied in relation to side lengths and triangles.
Trigonometry is a branch of mathematics. It is a form of knowledge and has no physical existence and so it has no appearance.
In Geometry, you learn about triangles. In Trigonometry you learn even more about triangles.