Well I would define a true/best friend like this:
* Someone I can trust
* Someone who genuinely likes and respects me for who I am
* Someone I can be honest and open with, and can share things with that are important to me
* Someone who understands me (or at least a part of it. Hell I don't even understand myself)
* Someone I can have good conversations with
* Someone I can count on
* Someone I feel comfortable with, and who doesn't make me feel like I need to prove myself
* Someone I trust enough to show my vulnerable side
* Someone I can have these comfortable silences with
* Someone who won't judge me
* Someone you're not spending time with because you need something of each other, but because you want to
I only know two people that meet all these criteria, but they are there, after all these years, so I know true friendship exists.
I'm very hesitant calling someone a 'friend', because friendships that don't give me what I need only cause me different levels of distress, and are usually too draining to me. The most important thing however is that it must be a two way process.
Then, of course, you also have other friends. The ones you hang out with once in a while but that don't really know me all that well, and thus aren't 'close' friends. I tend to keep them at a safe distance that feels comfortable for me. These friends come and go into your life, and I like them too, but it's a totally different relationship.