According to the official U.S. Senate Glossary a filibuster is an

Informal term for any attempt to block or delay Senate action on a bill or other matter by debating it at length, by offering numerous procedural motions, or by any other delaying or obstructive actions.

According to the AHD, a filibuster:

  1. a. The obstructing or delaying of legislative action, especially by prolonged speechmaking. 
  1. b. An instance of this, especially a prolonged speech.
  1. An adventurer who engages in a private military action in a foreign country.

Etymologically the English word filibuster derives from Dutch vrijbuiter, “pirate” via Spanish filibustero, or “freebooter”; the Spanish borrowed the word from French flibustier, who in turn derived their word from Dutch vrijbuiter.

English also derives our word freebooter “A person who pillages and plunders, especially a pirate” from Dutch vrijbuiter. Dutch vrijbuiter derives from from vrijbuit, plunder, a compound of vrij, free; (see prī- in the Appendix of Indo-European roots) + *buit, booty (from Middle Dutch būte, of Middle Low German origin).

The Proto Indo-European root * prī- “To love” is interesting; in addition to giving us Dutch vrij, prī- derivatives include friend, and Friday.

The record for the longest filibuster in the U.S. Congress is still held by Strom Thurmond’s 1957 24 hours and 18 minutes grandstand attempt to stop the Civil Rights act. You can read it, in all its shameful glory, in the Congressional Record Vol. 103 pt. 12: August 22 1957–August 30, 1957. .