Friendship is a term used to denote co-operative and supportive behavior between two or more humans. This article focuses on the notion specific to interpersonal relationships. In this sense, the term connotes a relationship which involves mutual knowledge, esteem, and affection 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 behavior, such as exchange of advice and the sharing of hardship. A friend is someone who may often demonstrate reciprocating and reflective behaviors. Yet for many, friendship is nothing more than the trust that someone or something will not harm them. Value that is found in friendships is often the result of a friend demonstrating 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.
In a comparison of personal relationships, friendship is considered to be closer than association, although there is a range of degrees of intimacy in both friendships and associations. Friendship and association can be thought of as spanning across the same continuum. The study of friendship is included in sociology, anthropology, philosophy, and zoology. Various theories of friendship have been proposed, among which are social psychology, social exchange theory, equity theory, relational dialectics, and attachment styles. See Interpersonal relationships

Friendship is considered one of the central human experiences, and has been sanctified by all major religions. The Epic of Gilgamesh, a Babylonian poem that is among the earliest known literary works in history, chronicles in great depth the friendship between Gilgamesh and Enkidu. The Greco-Roman had, as a paramount example, the friendship of Orestes and Pylades. The Abrahamic faiths have the story of David and Jonathan. Friendship played an important role in German Romanticism. A good example for this is Schiller's Die Bürgschaft. The Christian Gospels state that Jesus Christ declared, "No one has greater love than this, to lay down one's life for one's friends."(John 15:13).

In philosophy, Aristotle is known for his discussion (in the Nicomachean Ethics) of philia, which is usually (somewhat misleadingly) translated as "friendship," and certainly includes friendship, though is a much broader concept.

Cultural variations: (stub-section) A group of friends consists of two or more people who are in a mutually pleasing relationship engendering a sentiment of camaraderie, exclusivity, and mutual trust. There are varying degrees of "closeness" between friends. Hence, some people choose to differentiate and categorize friendships based on this sentiment.