The centrid is the cdentre of a group of points as cacuulated using the arithmetic mean of each of the Cartesian coordinates of the points. For example, in 2D sace, if the points have coordinates [ (2,0), (2,2), (3,1), (1,1) ] then the centroid is ( 2, 1 ), because 2 is the mean of the x coordinates: 2, 2, 3 and 1 and 1 is the mean of the y coordinates: 0, 2, 1, 1.
Used on Chap. 7: page 137; Chap. 9: page 195; Chap. 20: page 510