Evenness of lighting refers to how evenly and uniformly the light in a setting is distributed.  The human eye will automatically adjust itself to even lighting in a setting, even at very low light levels.  However, if an environment has uneven lighting the eye will not fully adjust and some areas will always appear darker than others.  Even lighting in an environment is generally created by having many lights spread out to cover an area, as opposed to having only a few very bright lights.