As you can see, there are as many definitions of "clean" as there are programmers (which means, according to my definition of "clean", that "clean" is not a very clean concept). Anyway, for me a "clean design" is one that exhibits, few extraneous dependencies (between its parts, between it and external modules, between the app and the OS, between the app and the phases of the moon, etc.). I.e. loose coupling.