A mathematical function is a type of equation that describes a relationship between two or more variables. The SINE function is a trigonometric function that is used to model periodic phenomena such as sound and light waves, the position and velocity of harmonic oscillators, and many other phenomena. It is defined as the ratio of the side opposite a given angle in a right triangle to the hypotenuse of the triangle. The SINE function is represented by the letter 's' in mathematics.