It's not that easy to do so, most people just define the word as what they look for in a friend. I personally think a friend is a person who will give and not expect something in return, a person who will stay with you no matter what the circumstance.
Not many people actually qualify as a friend under that definition. I'm not saying I'm perfect, but I try my hardest to be a 'friend'. My friends are my life, and each one is just as important to me.
But a true friendship can only happen when both sides work at it. Sure, a one sided friendship CAN work, but it's pointless. You try everything you possibly can to stay friends with this person, and this person acts like a leech and hardly does anything (IF they do anything) in return. It's a losing battle, and won't ever change. I've realized that, and I'm giving up on one sided friendships. It's not worth it.
As for a 'friend' that will stay around no matter the circumstance? Most people fail that test. They're 'friends' with you when it's convenient, but when it's not they drift away. I've noticed that most people are more of a 'friend' when I'm in school. It's convenient for them, I'm right there and they can easily talk to me. But in reality, I'm chronically ill. I DON'T make it to school most of the time, and that's not my fault. But when my 'friends' stop being my 'friend' when I'm not in school, that makes me feel horrible. I start thinking about how maybe the people I know would act different if I wasn't sick, and that's a horrible thing to think about. And it's true, the people I know WOULD act differently if I was "normal", or "healthy", and friendships shouldn't be like that. You should act the same no matter the circumstances, but lets get with the program... people are lazy and they go for what's convenient.
So, What's your definition of a 'friend' ?