We defined the sine and cosine functions as ratios of the sides of a right triangle in a circle. Since the triangle has 3 sides there are 6 possible combinations of ratios. While the sine and cosine are the two prominent ratios that can be formed, there are four others, and together they define the 6 trigonometric functions.
Points on Circles Using Cosecant, Secant, and Cotangent
Introduction to the unit circle | Trigonometry | Khan Academy
Secant (sec), cosecant (csc) and cotangent (cot) example | Trigonometry | Khan Academy