I have an algorithm that runs in O(n2), where n is the size of the problem. What does “the size of the problem” mean?

What will be an ideal response?

The count of something that is characteristic of the problem. Examples are Sorting: N is how many items there are to be sorted
Searching: N is how many items there are to be searched through
Compiling: N is the number of tokens in the program to be compiled.