What is a best friend? Are you a best friend?

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The day before yesterday, one of my best friends Lu --at least I think so-- took her best friend to play cards with me and also my mum. My mum said he was a nice guy to be her bf. But she thought he was not possible to be her bf, 'cause they knew each other a lot, and they're just best firends. Then I can't stop thinking, what is the meaning of "best friend", I cannot hit up one person / one friend to know me a lot, or who I understand so much~ Friendship is one mind in two bodies, is that true? So ,what do you think is a best friend?