Two or more people who share everything with one another, knowing they can be trusted. People who would, if needed, take a bullet for their friend(s). Two or more people who, through all the new friends, new hobbies, new jobs, rough times, and high school, are always there for eachother, no matter how busy they are. A best friend is someone who can make you feel better no matter what.
In some cases, the two or more part doesn't apply. It is quite possible to be your own best friend.
Once you find your best friend(s), hold on to them forever, for they are hard to find and oh so special.