What is best friend?

What is best friend? Is a best friend is same your close friend? Since he/she is best for you so he/she would be your closer friend too, I think there is no big different between the two. Or, on the other hand, best friend they call it a friend that you know since in your childhood, and since you know him/her very well and he/she is your old friend since before and you’re both close to each other then you treat him/her as a best friend, right? Or you just found a friend recently and you feel that you like him/her and you are comfortable with each other and you feel that he/she is good then she/he will be your best friend. Well I think it doesn’t work like that. You can measure a real friends or best friends during your tough times, calamities in life, sadness, burden and problems. Apparently, you will not know them during a happy time of your life, laughter, and happiness.