Bokeh, which is derived from the Japanese word, boke, that means "blur"; it is a photographic term used to refer to the way a lens renders the out-of-focus areas in an image. Defining "bokeh" is difficult since it is possesses a highly subjective quality but it is especially important for large-aperture lenses, macro lenses, and long focal length lenses because typically these produce a shallow depth of field, an effect that photographers use to help isolate the subject from a potentially confusing background.
Generally the shape of the aperture has a significant influence on the subjective quality of bokeh, because when a lens is stopped down to an aperture value other than its maximum (i.e. the minimum f/#), any out-of-focus points, particularly in bright highlight areas become blurred into the shape of the aperture rather than a perfect circle. In simple lens designs the aperture often has relatively few blades, five is typical, but this produces a regular polygonal shape that becomes clearly apparent in the out of focus areas. High quality lenses are designed with apertures that use nine or more blades to create a near circular aperture, which improves the appearance of the out of focus areas by forming a softer blur that makes them blend more smoothly.