There is no such thing. Do you mean definition? If so, it is the point at which the mass of an object can be considered to be effective/concentrated.
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In both cases, because of their symmetry, the center of gravity is in the geometric center.
If the cube is uniform ( ie it has uniform density) then the geometric center of the cube is its center of gravity.
Everything around you that has mass is an example of center gravity. :)
The center of gravity of a triangle is its centroid. The centroid of a triangle is the intersection of the three medians.
Just about where the center of the hole is.