1. In meteorology, generally, the interface or transition zone between two air masses of different density. Since the temperature distribution is the most important regulator of atmospheric density, a front almost invariably separates air masses of different temperature. Along with the basic density criterion and the common temperature criterion, many other features may distinguish a front, such as a pressure trough, a change in wind direction, a moisture discontinuity, and certain characteristic cloud and precipitation forms. The term front is used ambiguously for 1) frontal zone, the three- dimensional zone or layer of large horizontal density gradient, bounded by 2) frontal surfaces across which the horizontal density gradient is discontinuous (frontal surface usually refers specifically to the warmer side of the frontal zone); and 3) surface front, the line of intersection of a frontal surface or frontal zone with the earth's surface or, less frequently, with a specified constant-pressure surface. Types of front include polar front, arctic front, cold front, warm front, and occluded front. See also anafront, katafront, intertropical front, secondary front, upper front. 2. See wave front.