A ratio is a statement of a mathematical relationship comparing two quantities, often represented as a fraction. If we consider an angle A in a right angled triangle, we can construct various ratios to compare the lengths of the sides. Special relationships that exist in right angled triangles are called trigonometric ratios.
There are 3 basic trigonometric ratios that relate sides and angles together.
SinA = Opposite Side/Hypotenuse.
CosA = Adjacent Side/Hypotenuse.
TanA = Opposite side/Adjacent Side.