You already probably know those few parameters that are used to compare lenses and define their performance. But not many people know about bokeh. It is an interesting aspect of the lens and can tell something new about the optics and their design.
Bokeh is a Japanese word - the transliteration of a Japanese word for "blur" - describing the subjective aesthetic quality of out-of-focus areas of an image projected by a camera lens.
Here are a couple of articles that can explain this in details:

• "What is Bokeh?" by Ken Rockwell
• Understanding Bokeh by Harold M. Merklinger