Discs of light formed by the lens from points of light in a scene being photographed. The smaller the discs ( circles of confusion ) are, the sharper the image appears. When the circles of confusion can be seen as discs rather than points of light, that portion of the image is considered to be unsharp.Photo Tips
closely related to the above bokeh, the textbook definition is: the largest blur spot that is indistinguishable from the point source that is being rendered. Objects outside the depth of field of an image that the human eye can determine as “out of focus”.