Object Mode and Edit Mode
Places the pivot point at the averaged-out position of the selected items.
In Object Mode
In Object Mode, the Median Point is the averaged-out position of the origins of the selected objects. The shape and size of the objects is not taken into account.
Origins can be chosen freely and can even lie outside their object’s geometry, so that the Median Point is not always what you might expect.
In Edit Mode
In Edit Mode, the Median Point is the averaged-out position of the selected vertices. This means that the pivot point will shift towards the area with the densest geometry.
In the example below, the pivot point lies perfectly in the middle if both cubes have the same number of vertices, but heavily leans towards the side if one cube is subdivided – even though both cubes still have the same size.