Centroid

The "center of mass".

If you cut a shape out of a piece of card it would balance perfectly on its centroid.

Another way to think about it is the "average" position of all the points of an object.

Centroid of a Triangle

Where the medians intersect is the centroid. A median is a line segment from a corner to the midpoint of the opposite side.

Try moving points A, B or C: