Is it ensure, insure, or assure?
in crystallography, any of the ways in which the orientation of a crystal can be changed without seeming to change the position of its atoms. These changes may involve displacement of the whole structure along a crystallographic axis (translation), as well as the point group operations of rotation about an axis, reflection in a plane, inversion about a centre, or sequential rotary inversion. As demonstrated in the 1890s, only 230 distinct combinations of these changes are possible; these 230 combinations define the 230 space groups. A crystal can be assigned to one of these groups after the arrangement of its atoms is studied, as by X-ray crystallography, thereby providing a definitive way of categorizing the inherent symmetry of the crystal