で、ウィキペディアの中で見つけました。

Friendship is the cooperative and supportive relationship between people. In this sense, the term connotes a relationship which involves mutual knowledge, esteem, affection, and respect along with a degree of rendering service to friends in times of need or crisis. Friends will welcome each other's company and exhibit loyalty towards each other, often to the point of altruism. Their tastes will usually be similar and may converge, and they will share enjoyable activities. They will also engage in mutually helping behaviour, such as the exchange of advice and the sharing of hardship. A friend is someone who may often demonstrate reciprocating and reflective behaviours . Yet for some, the practical execution of friendship is little more than the trust that someone will not harm them.

Value that is found in friendships is often the result of a friend demonstrating the following on a consistent basis:

• the tendency to desire what is best for the other
• sympathy and empathy
• honesty perhaps in situations where it may be difficult for others to speak the truth, especially in terms of pointing out the perceived faults of one's counterpart
• mutual understanding