This document discusses trigonometric functions. It begins by explaining how trigonometric functions were originally used in ancient times for astronomical and terrestrial measurements but now model periodic natural phenomena. It then defines the six basic trigonometric functions - sine, cosine, tangent, cotangent, secant and cosecant - in terms of a unit circle. It provides the domains and ranges of each function and illustrates them using GeoGebra. It concludes by listing references used.