A filibuster is a parliamentary procedure in which one or more members of a legislative body prolong debate on proposed legislation so as to delay or entirely prevent a decision. It is sometimes referred to as "talking a bill to death" or "talking out a bill", and is characterized as a form of obstruction in a legislature or other decision-making body. A motion of cloture (also closure or guillotine) may sometimes restrict filibustering.

The term "filibuster" ultimately derives from the Dutch vrijbuiter ("freebooter", a pillaging and plundering adventurer), but the precise history of the word's borrowing into English is obscure. The Oxford English Dictionary finds its only known use in early modern English in a 1587 book describing "flibutors" who robbed supply convoys. In the late 18th century, the term was re-borrowed into English from its French form flibustier, a form that was used until the mid-19th century.

The modern English form "filibuster" was borrowed in the early 1850s from the Spanish filibustero (lawless plunderer). The term was applied to adventurers like William Walker who were engaging in private military expeditions in Latin America. Spain lost all its Central American territory in 1821. Over the course of the mid to late 19th century, the term "filibustering" became common in American English in the sense of "obstructing progress in a legislative assembly".

One of the first known practitioners of the filibuster was the Roman senator Cato the Younger. In debates over legislation he especially opposed, Cato would often obstruct the measure by speaking continuously until nightfall. As the Roman Senate had a rule requiring all business to conclude by dusk, Cato's deliberate long-winded speeches were an effective device to forestall a vote.

Cato attempted to use the filibuster at least twice to frustrate the political objectives of Julius Caesar. The first incident occurred during the summer of 60 BCE, when Caesar was returning home from his propraetorship in Hispania Ulterior. Caesar, by virtue of his military victories over the raiders and bandits in Hispania, had been awarded a triumph by the Senate. Having recently turned forty, Caesar had also become eligible to stand for consul. This posed a dilemma. Roman generals honored with a triumph were not allowed to enter the city prior to the ceremony, but candidates for the consulship were required, by law, to appear in person at the Forum. The date of the election, which had already been set, made it impossible for Caesar to stand unless he crossed the pomerium and gave up the right to his triumph. Caesar petitioned the Senate to stand in absentia, but Cato employed a filibuster to block the proposal. Faced with a choice between a triumph and the consulship, Caesar chose the consulship and entered the city.

Cato made use of the filibuster again in 59 BCE in response to a land reform bill sponsored by Caesar, who was then consul. When it was Cato's time to speak during the debate, he began one of his characteristically long-winded speeches. Caesar, who needed to pass the bill before his co-consul, Marcus Calpurnius Bibulus, took possession of the fasces at the end of the month, immediately recognized Cato's intent and ordered the lictors to jail him for the rest of the day. The move was unpopular with many senators and Caesar, realizing his mistake, soon ordered Cato's release. The day was wasted without the Senate ever getting to vote on a motion supporting the bill, but Caesar eventually circumvented Cato's opposition by taking the measure to the Tribal Assembly, where it passed.

Both houses of the Australian parliament have strictly enforced rules on how long members may speak, so filibusters are generally not possible, though this is not the case in some state legislatures.

In opposition, Tony Abbott's Liberal National coalition used suspension of standing orders in 2012 for the purposes of filibustering, most commonly during question time against the Labor government.