in legislative practice, the parliamentary tactic used in the United States Senate by a minority of the senators—sometimes even a single senator—to delay or prevent parliamentary action by talking so long that the majority either grants concessions or withdraws the bill.

Unlike the House of Representatives (Representatives, House of), in which rules limit speaking time, the Senate allows unlimited debate on a bill. Speeches can be completely irrelevant to the issue.

The word is derived from the Spanish filibustero (“freebooting”) and originally described piratical 16th-century privateers; it came into English usage to designate any irregular military adventurer, such as the Americans who took part in Latin-American insurrections in the 1850s. Filibuster was in use in the political sense by the mid-1800s. In 1957 Senator Strom Thurmond (Thurmond, Strom) of South Carolina talked for more than 24 hours, the longest individual filibuster on record, as part of an unsuccessful attempt by Southern senators to obstruct civil-rights legislation.

Invoking cloture on debate (i.e., limiting or ending a debate by calling for a vote) and holding round-the-clock sessions to tire the minority are measures used to defeat a filibuster.