A filibuster (from the Spanish filibustero), also known as a freebooter, is someone who engages in an unauthorized military expedition into a foreign country or territory to foster or support a political revolution or secession. The term is usually applied to United States citizens who incited insurrections across Latin America, particularly in the mid-19th century, usually with the goal of establishing an American-loyal regime that could later be annexed into the United States. Probably the most notable example is the Filibuster War initiated by William Walker in Nicaragua.

Filibusters are irregular soldiers who act without official authorization from their own government, and are generally motivated by financial gain, political ideology, or the thrill of adventure. Unlike mercenaries, filibusters are independently motivated and work for themselves, whilst a mercenary leader operates on behalf of others. The freewheeling actions of the filibusters of the 1850s led to the name being applied figuratively to the political act of filibustering in the United States Congress.