Bokeh

In photography, bokeh is the blur, or the aesthetic quality of the blur, in out-of-focus areas of an image.
Bokeh has been defined as "the way the lens renders out-of-focus points of light".
However, differences in lens aberrations and aperture shape cause some lens designs to blur the image in a way that is pleasing to the eye, while others produce blurring that is unpleasant or distracting.

Bokeh occurs for parts of the scene that lie outside the depth of field. Photographers sometimes deliberately use a shallow focus technique to create images with prominent out-of-focus regions.