Centre of Gravity (C.G.) is that point through which the resultant of a system of parallel forces formed by the weights of all particles of the body passes.

In other words, the point through which the whole weight of the body acts is known as centre of gravity.
Every body has one and only one c.g.

Centroid
The plane figures (like triangle, quadrilateral, circle, trapezoid, etc.) have only areas but no mass. The centre of area of such figure is known as centroid.

• It is also called the geometrical centre or the centre of gravity.

Derivation for Centre of Gravity

Derivation for the Centroid of Area

Axis of Symmetry
Axis of Symmetry is a line or axis which divides the given line, are or volume into two equal and identical parts.

Centroid always lies along the line of axis of symmetry.