datum point . An assumed or fixed point used as a reference from which calculations or measurements may be taken.

centered lattice (cen'-tered). A crystal lattice in which the axes have been chosen according to the rules for the crystal system, and in which there are lattice points at the centers of certain planes as well as at their corners; thus a centered lattice has two, three, or four lattice points per unit instead of one, as in a primitive lattice . Syn: multiply centered lattice. See also: base-centered lattice; one-face-centered lattice; face-centered lattice; body-centered lattice.

ice shelf . A sheet of very thick ice, with a level, gently sloping or gently undulating surface, which is attached to the land along one side but most of which is afloat and commonly bounded on the seaward side by a steep cliff ( ice front ) rising two to more than 50 m above sea level. Ice shelves have been formed along polar coasts (e.g., those of Antarctica, the Canadian Arctic islands, and Greenland), and they are generally of great breadth, some of them extending several hundreds of kilometers seaward from the coastline. They are nourished by annual snow accumulation and by seaward extension of land glaciers; limited areas may be aground. The term was used by Sir Douglas Mawson in 1912. Less recommended syn: barrier ice; shelf ice. Cf: ice barrier.

mesokurtic (mes-o-kur'-tic). Closely resembling a normal frequency distribution; a distribution curve that is neither leptokurtic (very peaked) nor platykurtic (flat across the top).