In greek ‘Trig’ means triangle and ‘metic’ is for measurement. The Trigonometric rations are used to express relationship between angles and sides of a right triangle ( a triangle with one 90° angle).
The non-right angle is usually designated by a greek letter θ (theta). The sides of the triangle are labeled according to the angles of the triangle.
The side opposite to right angle is designated as hypotenuse.
The side opposite to the angle θ is labeled as opposite side.
The side (other than hypotenuse) next to the angle θ is the adjacent side.
There are six trigonometric ratios of these three sides:
Sine, Cosine and Tangent are the basic trigonometric ratios. Cosecant, Secant and Cotangent are reciprocals (inverted fractions) of Sine, Cosine and Tangent respectively.
To remember the ratio for basic trigonometric ratios a very common mnemonic SOH CAH TOA is used.The mnemonic stands for:
Sine is Opposite over Hypotenuse
Cosine is Adjacent over Hypotenuse
Tangent is Opposite over Adjacent