In photography, bokeh (originally /ˈboʊkɛ/,^[1] /ˈboʊkeɪ/ boh-kay — also sometimes pronounced as /ˈboʊkə/boh-kə,^[2] Japanese: [boke]) is the aesthetic quality of the blur produced in the out-of-focus parts of an image produced by a lens.^[3][4][5] Bokeh has been defined as “the way the lens renders out-of-focus points of light”.^[6] 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—”good” and “bad” bokeh, respectively.^[7] 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.

Bokeh is often most visible around small background highlights, such as specular reflections and light sources, which is why it is often associated with such areas.^[7] However, bokeh is not limited to highlights; blur occurs in all out-of-focus regions of the image.

Source: Bokeh – Wikipedia