Consequential strangers are personal connections other than family and close friends. Also known as "peripheral" or "weak" ties, they lie in the broad social territory between strangers and intimates. The term was coined by Karen L. Fingerman and further developed by Melinda Blau, who collaborated with the psychologist to explore and popularize the concept.

Social life in the 21st century includes a wide array of personal connections, not just intimates—people associated with a particular part of one's life and daily activities, such as co-workers, neighbors, gym buddies, fellow volunteers and congregants, and providers of goods and services. Typically, peripheral ties far outnumber one's close relations. Decades of research have shown the importance of primary relationships in both psychological and physiological well being. Yet an analysis of the broader social landscape suggests that consequential strangers provide many of the same benefits as intimates as well as many distinct and complementary functions. They are not universally beneficial—undesirable consequential strangers who cannot be avoided can be found in the workplace, neighborhoods and organizations. But to thrive in a modern society, research suggests, it is vital to have a variety of connections.

The academic underpinnings of the concept can be traced back to Mark Granovetter's influential 1973 paper, "The Strength of Weak Ties," in which the sociologist argued that job opportunities were most likely to come from people in the more distant reaches of one's personal network—old bosses, former coworkers, college connections. Subsequent studies of social networks have shown that information, resources, and new connections—which confer social capital—are the province of weak ties in all arenas of life, not just employment. Benefits accrue to individuals, families, and the larger community as well. For example, one study found that African American mothers who made use of community ties as well as their families, were able to get their children into magnet schools and extracurricular programs that protected them from the dangers of inner city life.

Historically, relationship scholars have concentrated almost entirely on intimates: partners, children, parents, and, to a lesser extent, best friends. However, the trend toward urbanization underscored the importance of peripheral relationships. It is almost impossible to cope with the demands of modern life without weak ties. Thus, over the last several decades a handful of researchers and theorists have emphasized the important role of weak ties in anchoring individuals in the larger community. As one scholar observed in 1989, "Social scientists have long held that close and intimate friendships are the sine qua non of personal relationships, and no doubt they are important, but ties with acquaintances are equally important."

Technology has played a key role in the unprecedented rise of consequential strangers. Advances in transportation and communication have always altered the ways in which people can connect—the pony express allowed the printed word to spread; the telephone made it possible to speak to distant contacts. But computers and digitalization have accelerated the process. In 1997, when Merrill Lynch asked social observer Michael Schrage to analyze how "new" technologies would transform businesses, he stressed that the shift did not herald an "information revolution" as much as a "relationship revolution". His predictions proved prescient. Today, at least 1.6 billion people are connected via computer and mobile devices. Various social media, such as blogs, wikis, Twitter, SMS, and networking sites like Facebook facilitate contact, coordination, and collaboration across boundaries of time and space—and at minimal cost. Most relationships—personal and business—are now conducted both on and off the Internet. As a result, the ranks of consequential strangers have grown as has the ability to keep in touch with them.

Consequential strangers comprise the aggregate of personal connections outside one's inner circles of family and close friends. Such relationship are referred to elsewhere as "peripheral" (versus "core"), "secondary" (versus "primary"), or "weak ties" (versus "strong"). Colloquially, they are also known as acquaintances. But in reality, relationships cannot be neatly classified into groups. Rather, they fall along a continuum. Consequential strangers occupy the vast territory between strangers and close (or core) ties. They are just beyond Stanley Milgram's conception of a familiar stranger—one frequently encountered and broadly identifiable (a woman, an Asian, a cop)—but not known in the personal sense. In contrast, a personal and repeated pattern of interaction is evident with a consequential stranger.

The field of consequential strangers encompasses a diverse assortment of relationship types. The gradations between weak ties are often blurred, among other reasons because all relationships are fluid and dynamic. Over time, some consequential stranger connections become close friends or even intimate partners, while others stay at the level of acquaintanceship—for example, those "anchored" to a particular place, such as a school, gym, or train station or involved in a paid service or status hierarchies (e.g., a boss and worker).

Also, the vocabulary of relationships can be confusing. Whereas some languages, such as French, have two forms of the pronoun "you"—using the informal tu with intimates and the formal vous with acquaintances—English has no such markers. The word "friend" is used to describe close and casual relations. One must inquire further to find out what the speaker means. Likewise, "acquaintance" can be defined either as a relationship that falls short of friendship or as a stage from which the relationship becomes more intimate. Technology further complicates the linguistic issue. For example, Japanese sociologist Hidenori Tomita coined the term "intimate stranger"—a person with whom one shares intimate and yet anonymous contact—to describe "new relationships born through the new media."