This is not an exact definition, but it seems that intimacy and healthy relationships go hand in hand. Indeed, intimacy is basic in any meaningful relationship: the basis of friendship and one of the foundations of love.
The main forms of intimacy are emotional intimacy and physical intimacy. Intellectual intimacy, familiarity with a person's culture and interests, is common among friends. Members of religious or philosophic groups may also perceive a "spiritual intimacy" in common.
Terms for partners in various degrees of love include
Intimacy Facts for Kids. Kiddle Encyclopedia.