Definition of centers:
(n) :
The point in the interior of a circle that is equidistant from all points on the circumference.
(n) :
The point in the interior of a sphere that is equidistant from all points on the circumference.
(n) :
The middle portion of something; the part well away from the edges.
(n) :
(geometry) The point on a line that is midway between the ends.
(n) :
(geometry) The point in the interior of any figure of any number of dimensions that has as its coordinates the arithmetic mean of the coordinates of all points on the perimeter of the figure (or of all points in the interior for a center of volume).
(n) :
(geometry) A point on a triangle's plane that is in some sense in its middle.