Best friend
A Best Friend is usually a chosen friend with whom one shares a deeper level of understanding, trust and affection than most others they are close to. A best friend may be of the same sex or of the opposite sex. Often when a best friend is of the opposite sex, it is more difficult to keep the friendship because of jealousy of each other's boyfriends and girlfriends.
The term is usually only used by children, in adulthood one may see all their friends as equal. However if they are especially close to someone who they are not romantically linked, they will not openly acknowledge them as a 'best friend'.
Best friends usually share and enjoy the same activities. In fact, they often meet while doing an activity that they enjoy and this is the common ground on which the friendship is founded. Best friends often trust each other with information that they would not share with anyone else. This is a bond of friendship that, once broken, is hard to regain.
Someone who is a friend of a particular person who has another 'best friend' may become jealous of that person, particularly if the person is older and different genders are involved.
Throughout history, in Western Culture, a best friend has meant different things. In the twelfth through the fourteenth centuries, male friendships were seen as the closest relationship one could have. Females were believed to be inferior to males, so they were not able to have truly meaningful relationships. As a result, adult men had very close friendships in order to have fulfilling relationships that they couldnt get through marriage. Women were not considered able to have best friends because of their inferiority.