Difficulty: What's it mean to you?

I was having this interesting conversation earlier today with someone about the definition of difficulty.

I always believed something was harder if it meant that the next tier of things you learn require more skills and require you to learn more things.

He believed that diffculty meant that the next tier required a better mastery of earlier ideas, so any problems you had with earlier material would eventually catch up with you, and as a result would make the next tier impossible to master.

For example: if you were learning math, he believes that an earlier tier, such as learning division, would create huge problems later if you had trouble learning it then. So when you did algebraic divison, it would catch up with you.

I always believed it meant that the next tier, IE calculus would require more skills,and better insight in order to comprehend it.

What do you guys think? How do you define something hard?

Jam it back in, in the dark.