Dictionary.net

(Math.) If an angle is supposed to be at the base of a right-angled triangle,its trigonometrical functions are the ratios of the sides ; viz. the sine, the ratio of perpendicular to hypotenuse ; the tangent, the ratio of perpendicular to base ; the secant, the ratio of hypotenuse to base ; the cosine, cotangent, cosecant, are the same function of the complement of the angle. The definitions apply strictly to an acute angle only, but they admit of extension to angles of all magnitudes. There is another and an older way of defining these functions, according to which they are treated as lines, and called the Trigonometrical lines.