In mathematics, the<unk>  (also called circular<unk>  are<unk>  of an angle. They are important in the study of triangles and modeling periodic phenomena, among many other applications.<unk>  are commonly defined as ratios of two sides of a right triangle containing the angle, and can equivalently be defined as the lengths of various line segments from a unit circle. More modern definitions express them as infinite series or as solutions of certain differential equations, allowing their extension to arbitrary positive and negative values and even to complex numbers.
In modern usage, there are six basic<unk> , which are tabulated here along with equations relating them to one another.  Especially in the case of the last four, these relations are often taken as the definitions of those<unk> , but one can define them equally well geometrically or by other means and then derive these relations.
Trigonometric functions
Dirac delta function
Acoustic reflex
Concussion classification
Optokinetic reflex
Otalgia classification
Cantilever mechanics 
Refractive index