How do you define friendship?

Lately I've been wondering what makes someone a friend. For my whole life, I've always considered someone a friend if I talk to them regularly and voluntarily. For example, if I have a class with someone and I talk to them every day in that class, and we typically have good conversations, I'd call them a friend. If we hang out outside of where we met, then we're good friends. But is that a real friendship? Just hanging around people that are entertaining? Aristotle said something about how a friend is just one soul in two bodies. I've been wondering how deep friendship is supposed to go. Really, there's three people in my life that I'd put up for consideration as "true" friends. They're the people I care the most about in this world. But I've never opened up to them. We hang out all the time, we've had deep conversations, and been through thick and thin, but... I wonder if you could call it a real friendship. Makes me wonder if I have anyone that an outsider would call a friend. Or if I'd ever be able to reach a level of trust that some people seem to talk about when they talk about friendship. Cause, really, when I'm down, I don't call anyone. But there are people in my life that I'd die for. How would you define friendship? Is just casual socializing enough? Or do friends have to trust each other with their lives? Have you ever had someone like that? What was it like?