Gravity
The
center of Gravity can be defined as the point where the weight of the all body
depends is called Center of Gravity or Center of
mass
Examples
1. When you throw a ball up in the air, it always comes back down.
2. When you jump rope, the rope doesn't stay
above our heads in the air.
3. When you fall, gravity pushes you to the
ground resulting in bruises, scrapes, and burns.

Reference
http://wiki.answers.com/Q/What_is_the_definition_of_''centre_of_gravity''