Bokeh
The aesthetic character of the out-of-focus areas of an image, as distinct from how much of the image is out of focus.
In depth
From the Japanese boke, meaning blur, the word describes quality rather than quantity. Smooth bokeh renders defocused highlights as evenly filled discs with soft edges; harsh or 'nervous' bokeh gives them bright rims and doubled outlines. It is governed by spherical aberration correction, aperture blade shape and the number of elements, and it is one of the few lens qualities that cannot be measured on a chart.
Examples in practice
Two 85 mm lenses at f/1.8 blur the background by the same amount, but one renders distant fairy lights as creamy discs and the other as bright-edged rings.