William Magear "Boss" Tweed was an American politician most notable for being the political boss of Tammany Hall, the Democratic Party's political machine that played a major role in the politics of 19th-century New York City and State.

In politics, a boss is a person who controls a faction or local branch of a political party. They do not necessarily hold public office themselves; most historical bosses did not, at least during the times of their greatest influence. Numerous officeholders in that unit are subordinate to the single boss in party affairs. Bosses may base their power on the support of numerous voters, usually organized voting blocs, and manage a coalition of these blocs and various other stakeholders. When the party wins, they typically control appointments in their unit, and have a voice at the higher levels. Reformers typically allege that political bosses are corrupt. This corruption is usually tied to patronage: the exchange of jobs, lucrative contracts and other political favors for votes, campaign contributions and sometimes outright bribes.