Friends are people who
- spend time together
- mutually want to spend time together
- support each other
Often times, it's difficult to pin down exactly what makes a person a friend or what doesn't. However, I've found that these three criteria are remarkably useful for differentiating between different types of relationships.
For example, (2) highlights the ways in which a classmate or a coworker is different from a friend. In a work/class environment, you're going to spend a lot of time with people and you're going to be supporting each other, but keep in mind, that's all a part of the job. Thus a friend is more than just someone who's supportive and spends time with you; they're someone who wants to do those things.
(1) also helps differentiate between people who could/want to/used to be friends and people who actually are. Two people might have been friends at one point, but when neither spends time with the other anymore, it calls into question whether they still are.
(3) of course, is what makes the difference between the sunny-day friend and the rainy day friend. If someone's truly your friend, they'll be there to support you during rough times.