What is a friend?

And does this change over time?
What I am driving at here is simply that I am sure we all perceive friendship slightly differently and based upon our personal expectations and experiences, may have cause at some time to reflect on this question.
Are those people that you thought were your friends STILL around when you most need them, or are they conspicuous by their absence in your life now you could do with some support and anchors?
I wonder what your experience has been :-)?