Average case complexity of Algo

What is meant by average case complexity of an algorithm?

  1. Average of complexity of all possible complexities over different inputs, OR
  2. Complexity of an input which is average of all possible inputs.

According to me when we say that average case complexity of Quicksort is N log(N), I understand that it is for an input which is not skewed towards being already somewhat partially sorted nor being extremely unsorted. This complexity is for a random input, which (according to me) means an input which is average of all possible inputs. Hence the term average complexity.

Kindly correct me where I am wrong. And what average complexity actually means.