Trigonometry is a branch of mathematics that deals with triangles. Trigonometry is derived from Greek words "trigonon" means triangle and "metron" means measurement. Thus, trigonometry deals with measurement of triangles.
Right Angled Triangle
To define Trigonometric function, consider a right triangle XYZ with angle $\theta $ as shown in the figure above.
If $\theta $ is taken as an angle of reference,
1) YZ the side opposite to $\theta $ is called the perpendicular, in this case side "a".

2) XY the side opposite to the right angle is called the hypotenuse, in this case side "h".
Hypotenuse is always the longest side of a right-angled triangle.

3) XZ the third side is called the base, in this case side "b".
The Trigonometric Functions are:
Trigonometry Functions

Pythagoras' Theorem:
In a right angled triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides.
$a^{2} + b^{2} = c^{2}$