WASHINGTON (AP) - Filibusters let minority parties tie the Senate in knots and send majority parties through the roof with frustration. But they seldom involve exhausted senators speaking endlessly on the chamber's floor. Instead, filibusters, taken from a Dutch word that means "pirate," involve any delaying tactic that blocks the Senate from voting on legislation or a nomination. There is no universal agreement on what a filibuster is. It can involve speaking, forcing repeated votes, or even a threat or perceived threat to block a measure. Filibusters are not in the Constitution. They flow from the Senate's loose rules, which impose few restrictions on debate. The most important thing about filibusters is they take support from three-fifths of all senators to halt. In the 100-member chamber, that means the votes of 60 senators are needed. Unless there is some bipartisan consensus, that can be a tough margin for today's majority Democrats because they control just 54 votes, including two usually supportive independents. Without a filibuster, approving legislation or nominations requires a mere majority. The longest filibuster was when Sen. Strom Thurmond, then a Democrat from South Carolina, spoke for 24 hours 18 minutes against a 1957 civil rights bill that eventually passed. In March, Sen. Rand Paul, R-Ky., spoke for 12 hours 52 minutes opposing President Barack Obama's nominee to head the CIA, who was later approved. More famous than both: Actor Jimmy Stewart's filibuster in the 1939 movie, "Mr. Smith Goes to Washington."