Originally, the names given to the 6 possible ratios between pairs of sides in a right-angled triangle (sine, cosine, tangent, cotangent, secant, cosecant). Usually the triangle is drawn with resting on one of its shorter sides, and these functions are viewed as depending on the bottom acute angle (angle smaller than 90 degrees). Later the definitions were extended for any angle, using the unit circle. Though initially introduced as a tool of land-surveying, today trigonometric functions play a key role in many areas of mathematics.

sin == opposite hypotenuse cos == adjacent hypotenuse tan == opposite adjacent cot = tan = sec = cos = csc = sin = sin = cos = tan = cot = tan = sec = cos = csc = sin