A stranger is a person who is not known by the people of a particular social milieu, a grouping of people that can be large or small. The word may only imply a lack of intimacy, such as a neighbor who is recognized but is not a friend or acquaintance, and is not part of a group. In kinship-driven societies, the stranger who follows basic customs may be welcome, and indeed fall into a strong social role of guest.
It may refer to one who is unknown and alien, a foreigner who is suspect, or a newcomer to be welcomed to a community. "Alien" may, in a given context, imply even more than strangeness, such as a lack of common language, or, in science fiction, being from another species.
In Greek tragedy, according to the Greek playwright Sophocles, Oedipus in the drama Oedipus the King did not know his father, who appeared to him as a stranger along the road, and during a quarrel, killed his own father without realizing it.
In the American West of the 19th century, "Hello, stranger" was a neutral greeting in a saloon, often followed by an offer of a drink.
The adjective "strange" and the noun "stranger" may be applied to the behavior of one who is otherwise known. Teenage fads often are called strange by parents and their peers, and a frustrated parent may say of a rebellious child, "he is a stranger to me." In cultures that practice ostracism, unacceptable exogamy, or marriage outside the group, may cause a parent to tell a child "you are a stranger to me," and even formally mourn for the child as if he or she had died.
The author Albert Camus wrote a novel called The Stranger which expressed the existentialist philosophy which is about a man who is strange to himself as well as others, who murders another man seemingly arbitrarily, is tried and convicted, and ultimately sentenced to death.