The quality of the out-of-focus area in a wide-aperture image is called bokeh, originally from the Japanese word boke, pronounced bo-keh, which means fuzzy. In photography, bokeh reflects the shape and number of diaphragm blades in the lens, and that determines, in part, the way that out-of-focus points of light are rendered in the image. Bokeh is also a result of spherical aberration that affects how the light is collected. Although subject to controversy, photographers often judge bokeh as being either good or bad. Good bokeh renders the out-of-focus areas as smooth, uniform, and generally circular shapes with nicely blurred edges. Bad bokeh, on the other hand, renders out-of-focus areas with polygonal shapes, hard edges, and with illumination that creates a brighter area at the outside of the disk shape.
Source: wiktionary