Sine, Cosine and Tangent are the main functions used in Trigonometry and are based on a Right-Angled Triangle. Before getting stuck into the functions, it helps to give a name to each side of a right triangle: Opposite is always opposite the angle. And Adjacent is always next to the angle.
Sine is an entire function and is implemented in the Wolfram Language as Sin [z]. A related function known as the hyperbolic sine is similarly defined, The sine function can be defined analytically by the infinite sum. It is also given by the imaginary part of the complex exponential.