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A maven (also mavin) is a trusted expert in a particular field, who seeks to pass knowledge on to others. The word maven comes from Hebrew, via Yiddish, and means one who understands, based on an accumulation of knowledge.
The word reached English through Yiddish meyvn, from Late Hebrew mēbhīn, which in turn derived it from the Hebrew mevin (מבֿין), meaning "one who understands," and relates to the word binah, which denotes understanding or wisdom in general. It was first recorded (spelled mayvin) in English in 1950 (in the Jewish Standard of Toronto), and popularized in the United States in the 1960s by a series of commercials created by Martin Solow for Vita Herring, featuring "The Beloved Herring Maven." The “Beloved Herring Maven“ ran in radio ads from 1964-1968, and was then brought back in 1983 with Allan Swift, the original voice of the Maven.
Many sites credit Vita with popularizing the word maven. An example of a print advertisement including the Maven is: "Get Vita at your favorite supermarket, grocery or delicatessen. Tell them the beloved Maven sent you. It won’t save you any money: but you’ll get the best herring".
Since the 1980s it has become more common since William Safire adapted it to describe himself as "the language maven". The word is mainly confined to American English, but did not appear with the publication of the 1976 edition of Webster's Third New International Dictionary; it is, however, included in the Oxford English Dictionary second edition (1989). Numerous individuals and entities now affix maven or mavin to indicate their expertise in a particular area.
Malcolm Gladwell used the term in his book The Tipping Point (Little Brown, 2000) to describe those who are intense gatherers of information and impressions, and so are often the first to pick up on new or nascent trends. The popularity of the work of Safire and Gladwell has made the word widely used in their particular contexts. Gladwell also suggests that mavens may act most effectively when in collaboration with connectors - i.e., those people who have wide network of casual acquaintances by whom they are trusted, often a network that crosses many social boundaries and groups. Connectors can thus easily and widely distribute the advice or insights of a maven.
In the afterword of The Tipping Point, Gladwell described a "maven trap" as a method of obtaining information from mavens. In the book he gave the example of the toll-free telephone number on the back of a bar of Ivory soap, which one could call with questions or comments about the product. Gladwell's opinion is that only those who are passionate or knowledgeable about soap would bother to call and that this is a method by which the company could inexpensively glean valuable information about their market.
Mars Atmosphere and Volatile EvolutioN (MAVEN) is a planned space exploration mission to send a space probe to orbit Mars and study its atmosphere. It will help determine what caused the Martian atmosphere —and water— to be lost to space, making the climate increasingly inhospitable for life.
In network theory and sociology, a maven is someone who has a disproportionate influence on other members of the network. The role of mavens in propagating knowledge and preferences has been established in various domains, from politics to social trends.
In popular culture 
Michael Chabon's 2007 noir-ish alternate reality novel The Yiddish Policemen's Union contained a pivotal character called "The Boundary Maven", whose knowledge of orthodox Jewish rules regarding the ability of people to legitimately walk within their own property on the Sabbath allowed him to use string between lamp-posts to create expansive "personal" boundaries for those willing to pay him.
In the video game League of Legends, champion Sona's title is "Maven of the Strings." The in-game lore depicts her as a mute musician who is an expert in playing music, and wishes to teach others about it.