center of inversion
center of inversion[′sen·tər əv in′vər·zhən]
A point in a crystal lattice such that the lattice is left invariant by an inversion in the point.
The point O with respect to which an inversion is defined, so that every point P is mapped by the inversion into a point Q that is collinear with O and P.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.