Difficulty: What's it mean to you?

I'd say difficulty is used in the developement of skill, the matter of difficulty are the degrees between your current skill in an area and how much more skill it would require to master a task you're not yet able to master without doing some effort.
Therefore difficulty is the matter of effort you'll have to do finishing a task compared to the ideal effort which would be the smaller amount of effort it would take for someone who "completely" (which is a rather idealistic belief) masters finishing the task.
For example: It is more difficult for a pianist who's at a lower level to play Liszt's Hungarian Rhapsody no. 2 than someone who's already at a higher level of skill and is playing far more difficult pieces (of whom would probably easier again for someone who's even more skillfull in that area).

Jam it back in, in the dark.