Friendship is a relationship of mutual affection between people. Friendship is a stronger form of interpersonal bond than an association. Friendship has been studied in academic fields such as communication, sociology, social psychology, anthropology, and philosophy. Various academic theories of friendship have been proposed, including social exchange theory, equity theory, relational dialectics, and attachment styles. A World Happiness Database study found that people with close friendships are happier.

Although there are many forms of friendship, some of which may vary from place to place, certain characteristics are present in many types of bond. Such characteristics include affection; kindness; love; virtue; sympathy; empathy; honesty; altruism; mutual understanding and compassion; enjoyment of each other's company; trust; and the ability to be oneself, express one's feelings, and make mistakes without fear of judgment from the friend.

While there is no practical limit on what types of people can form a friendship, friends tend to share common backgrounds, occupations, or interests and have similar demographics.

"One of the most beautiful qualities of true friendship is to understand and to be understood."

Lucius Annaeus Seneca