One of the papers at the York-Norwich conference was on Friendship. Now, that person would have to be the one to tell you all the theory and texts behind it, but I just need to say that a friend is someone who will go dig through their academic library to find you some obscure article or resource you don't yourself have. And then will take pictures of it or scan it, and send it to you.
There's lots of other ways to define friendship, all totally valid. But once someone has done that for you, there's no denying.
A friend also encourages you when you're down, helps you clarify your thoughts, critiques drafts, suggests books, and tells you not to give up.
I have fabulous friends. Thanks, guys.