What's a learning curve and why is steep not hard?

Question

• What exactly is a learning curve?
• And why is it wrong to use the term "steep learning curve" for something which has high entry barriers and takes quite some time to get into?

As to the why-ness of this question:

• The terms are used often and inconsistently on Stack Overflow
• I myself have been confused by it
• Mostly the newbies are confronted with these terms when they for example ask questions like "what's the best php development framework"

Answer 1

It's a curve of time versus proficiency.

Steep for hard is wrong because it'd mean that you get very proficient in very little time

proficiency
  |   __
  |  |
  |  |    Proficient in little time (steep = easy)
  |  |
  |_/____________
       time

proficiency
  |
  |       Proficient in lots of time (gentle = hard)
  |            __             
  |           /
  |__________/___
       time