Friendship, Defined?

Close friends. The people you could not live without, who know you better than you know yourself. They're the ones you know will always, always be there for you even if you mess up--especially when you mess up--even if the world around you feels completely messed up. They're there. They're the people you depend on. The people you know will never walk out on you.

For the past few years, I've defined that type of friendship with the quote, "It's the friends you can call at 4 a.m. that matter." I defined those friendships at least partially based on how much time I spent with those friends (in person and on the phone), on whether I could call them whenever I needed them.

But one of those close friends recently challenged that definition. She said she doesn't think those relationships are defined by the long lengths of time you spend with your close friends or by whether you can call them at 4 a.m. She views that relationship in an entirely different way.

But then again, what are close relationships deprived of those things?
What makes a friend into the sort of friend that knows everything about you, the sort of friend who you can't imagine losing touch with? And what makes keeps them as that sort of friend?