The word trigonometry originates from the Greek for triangle and measure. Hence, it deals with the measurements of triangles such as its sides and angles. The most important trigonometric functions are sine, cosine and tangent. Consider the following right-angled triangle.
Sine, cosine and tangent are defined as follows:
Sine (sin) of (alpha is the angle between the hypotenuse and adjacent) is defined by the ratio of the opposite to the hypotenuse.
Cosine (cos) of is defined by the ratio of the adjacent to the hypotenuse.
Tangent (tan) of is defined by the ratio of the opposite to the adjacent or by the ratio of sine to cosine.