Could someone explain to me what "best Friend" means?

I see it in various places. And to see it here where people....even those who are married.... claim that some opposite sex friendship is their "best friend." Maybe I am a very cautious person. Friends are great to have. But I would not want to burden anyone with the rank of "best friend" when they may not at one point deliver. For those who use the expression "best friend" tell us what it means to you.... and whether there is a difference when the friend is of the opposite sex....... and anyone else who has an opinion on that expression.....

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