What The Heck is a BFF?

I met my first best friend in kindergarten by raising my hand and yelling excitedly in front of the class "Who wants to be my friend?" Not a second later did a girl jump up and respond "Me!" This girl and I live less than 4 minutes away from each other, we attend the same school, and are in almost every single class together. We probally say a total of 150 words to each other each week. When we were younger, we promised each other we would be at the other's wedding. I highly doubt that we will stay in contact when we go off to college. So I'm asking, what the heck is a BFF? Is it someone you exchange awkward smiles with in the hallway? Someone you go shopping with on the weekend? I have people I do both these things with. I wonder sometimes, "Will I call this person a friend in a year or so?" "Is this someone I'm gonna take with me to help pick out my wedding gown?" My parents had "best friends." My father's best man was a guy my dad was very close to, yet I have never met this man. He doesn't come over to our house on Sunday and watch football with my dad. My mom has had better luck with "best friends." She stays in touch with a few people from her youth. This doesn't offer me much hope with staying in touch with friends when I am older. Even today, I'm wary of this one friend who I can't say we will be close when we grow up. I want to very much stay in touch with her. We made plans to get matching tattos when we are old enough. But I still feel insecure. We have gotten into little fights recently. I'm probally just being a worry wort, but this is one friendship I value. I've considered her my BFF. And visa versa. I'm just skeptical if we will continue holding those titles for each other. So really, what is a BFF? Can you really have a best friend forever?