A familiar stranger is an individual who is recognized from regular activities, but with whom one does not interact. First identified by Stanley Milgram in the 1972 paper The Familiar Stranger: An Aspect of Urban Anonymity, it has become an increasingly popular concept in research about social networks.
Somebody who is seen daily on the train or at the gym, but with whom one does not otherwise communicate, is an example of a familiar stranger. Interestingly, if such individuals meet in an unfamiliar setting, for example while travelling, they are more likely to introduce themselves than would perfect strangers, since they have a background of shared experiences.
The 1972 paper was based on two independent research projects he conducted in 1971, one at CUNY and the other at a train station. Milgram published a second paper on the subject, Frozen World of the Familiar Stranger, in 1974. It appeared in the magazine Psychology Today.
UC Berkeley has adopted the concept as part of a research program titled Familiar Stranger Project at its Intel Research Laboratory.