At an abstract level, it’s an everlasting bond which exists for its own sake and all that yada.
But it’s not so easy to define at a more practical level.
Sometimes you share a lot of wonderful moments with a someone who seems to be just the sort of person who’d make a great friend…. but it somehow never happens.
Sometimes you put up with mental third-degree, hard times, emotional strain with someone you wouldn’t really care for if you met them just now, all because you call that person a friend.
Do friends get cut out of your life? When would you do that? Is it a conscious decision you take, or is it something that gradually happens on its own? Is it not being calculative? Rather, is being calculative such a bad thing as it is made out to be?
People change. Relationships change. Does that go for friendships too?
Just what is it that holds friends together? Just knowing each other reasonably well? Or just calling it friendship?
In a reader
The NITK Numbskulls Page 
The Selfish Gene